Year, 50 year old nuts can give you lots of problems.
Potential interior rust in a steel frame.
Stressed steel frame.
No longer supported head tube sizes, seat tube sizes, nut sizes, free wheel tools, bottom bracket bearings, lock wrenches, cable and tubing sizes.
Not that this affects her bike, but finding parts for your cassette, your dérailleurs, etc.
A lot of things in bikes today make way more sense that that old school stuff. . .integrated headsets, sealed bearings, external bearings. A lot of that isn’t just marketing, but things that make the bike easier to work on, and less prone to failure.
Integrated shifting makes for easier and safer riding.
Again, those old Schwinns are about some of the most reliable machines mankind has ever put together. Don’t let them scare you off with things like this.
2 weeks to get a new nut? Unless you absolutely insist on keeping every single nut and washer 100% original equipment you shouldn’t have any problem with the replacement of simple nuts and washers. I know I’ve always been able to find replacement nuts in a bucket full of random nuts and washers when fixing up old bikes.
Many times it’s even possible to cannibalize parts off other bikes like bearing cones, cups, and balls. Bikes are pretty standard in many cases.
The only thing you may have trouble getting would be things like original equipment hand grips, seat, fenders - things like that. It sounds like the bike you fell in love with already has these thing (which is the reason for the high price tag I’m sure) so that isn’t a problem. If you don’t insist on original equipment then nearly everything on the bike can fairly easily be replaced.
yeah, one of the bike shops showed me a catalog full of different cruisers I could order, which actually looked pretty good. But I don’t want one of those. They lack the authenticity of a true vintage bike, they have gears (I know, who the hell would want different gears on their bike?) and I don’t dig the beach cruiser-type handlebars (high and wide) that they come with, so I’d have to order new handlebars too. Plus, they were all around $300 without the new handlebars and fenders I’d have to order to really get the look I wanted.
I had my old bike for several years and never had major mechanical issues, and I like original parts but I’m not a stickler for that, so I would be ok replacing a part with something new. And I’m perfectly comfortable dealing with 50 year old nuts. 
But I bought this beauty for $200, plus a few accessories (I still need to get a seat cover). It’s got the style I want, it’s in great working order (much more so than my old bike, the humidity and summer rain had rusted the chain and spokes) and it’s cheaper than the pretty in pink one. Not quite so girly, but that might be fun. Maybe I’ll see if I can get/make black and red tassels (you can’t quite tell in the picture but it’s black with red accents) and put some skulls on the valve stems. A tough girly bike.
And it’s 70 years old. This bike survived a world war. And I just think that’s cool.
If you love the bike, don’t let the price scare you off. Go for it! How often do you find a bike that you fall in love with? Get a better lock this time, though.
I just spent $595 for a bike. I jumped at the chance. It’s a Rans recumbent Tailwind. It has about 9 miles on it, and it cost $1000 new.
You shouldn’t unless it’s this bike:
http://www.uwec.edu/DC/AP/IFS/pastfilms/peewee.html
Heh. I’d have said that anything less than $200 for a bike sounds like a bike I wouldn’t want to spend much time on. But, then, it’s a whole different class of bikes. I think it’s a waste of money to buy a bike that’s heavy, slow, and has only one gear, but if it makes you happy and you can afford it, then that’s what it’s for.
Apart from cancer having completely rusted one of these frames away I just don’t see any problem with the structure of the frame being a problem. I TRIED to break a 72 Schwinn Suburban once and this thing had been left in weeds in the back of someone’s shed for YEARS in all the harsh elements of Wisconsin’s seasons. These things are big, heavy and damn near indestructible. My '72 Suburban had a great deal of surface rust and the paint was faded away. You could barely make out the decals anymore.
It would take quite a few years to wear out something like a seat tube. 
Sure they can bend - I had it happen once and nearly lost my ability to reproduce, but it happened once and only once. Headsets, bearing sets? I’ve never seen a bearnig set so wretched and gritty that it couldn’t spend a few nights in an oil bath and a little scrubbing that didn’t make them serviceable again. They are normally pitted and scarred, but will still spin freely if re-packed and assembled. Any bearing balls so badly pitted and scarred can be replaced. It’s not like bearing balls aren’t made anymore. The cups and cones can get pitted and damaged too, but again not so much as you would notice while pedaling a 40 pound bicycle.
You aren’t going to be using one of these bikes in a race anyway.
Most of the time severe damage happens to bearing sets when someone installs the bearing cage backwards. I’ve seen that a few times. Something like that can cause you problems if you can’t find a replacement.
Cables? Unless you’re talking about pre war bikes I’ve never seen any other cable used than the standard. Many bikes from the 40’s and 50’s have coaster breaks on the rear hub only anyway. Some old bikes use things like linkages to operate drum brakes. Unless you have something really unusual cables aren’t a problem.
Freewheels can be dissassembled with use of a chain whip and pipe wrench. I Don’t know why you would need to though. If you ever had to do this it may just be easier to get a whole new hub with a coaster brake they are still made today.
Chain - Standard, still used to this day.
Spokes - not a problem.
Any well-equipped bike (or backyard) mechanic should have all the tools necessary to work on old Schwinn bicycles. And you could likely do most of the work yourself.
If the fact that the bike you are looking at only has a single speed there should be no problem with finding a three speed hub that would work beautifully with this bike and easily installed too. Hell I put a modern 7 speed Sachs hub on an old schwinn just for the hell of it with no problems. It used the same chain size.
Get youself an old bike. You will love it.
I know this is going to sound arrogant, but I remember when they auctioned off Pee Wee’s bike about 10 years ago. This was back when I had a lot of spare cash. I was seriously going to bid on it. If I recall correctly, it ended up going for $8000, and I remember thinking that I should’ve bought it. Ridiculous you say? Well, it’s probably the most famous bike in movie history, surpassing that of The Bicycle Thief and the funky old 10 speed in Breaking Away. First of all it had the provenance and secondly it is instantly recognizable. I suspect it would have been a good investment in movie memorabilia.