unclviny, from what you describe, you dont want a web designer, you want a web developer.
If your site was pretty much static html pages with little functionality, you would be best served using a web designer, who could contract out any functionality to a developer. But with the amount of functionality you listed, you should do it the other way around: get a web developer who will contract out the graphic design to someone else. If youve been talking to people who give you the “Im an Artist” line of BS, youve been talking to the wrong people for the amount of functionality you want.
With that amount of functionality, you should be talking to and dealing with the person/people who will be writing code, not the person/people who will be making the images. Web site makers roughly fall into two groups; those run by developers who hire/delegate to graphic artists, and those run by graphic artists who hire/delegate to developers. Very very rarely will you find people who excell at both; so rarely its not worth spending the time trying to find one.
As for the relationship thing, they are going to push that because, well, its easy money. That, and for what youre asking for, its obvious that updates or at least a certain amount of maintenance is going to be needed.
You are correct though; its very possible to write code in such a way that pretty much any future developer should be able to go in, figure out how its set up and do any tasks that need be done. In other words, you should be able to have someone do a one-off for a lump sum and throw the maintenance work to any decent developer you can find when you need it done.
But in the real world Web site builders, especially the smaller ones, can be just as untrustworthy as, say, car mechanics. In fact, I suggest you approach the whole thing just exactly as you would getting your car fixed. Shop around, get quotes, ask around about reputation, very very explicit contracts as to what will be done vs what wont, etc. I will say that if a web builder wont agree to work with you ~without~ a relationship, dont let the door hit them in the ass.
Oh, and the poster who said about 3-5k is what to expect to pay is pretty much right; you could probably get lower by going to a one person shop though, which might be one thing to consider: get the code of the site built first, get the functionality working how you want, which should be very doable by a one-developer shop. Then find a graphic artist to deal with the look and feel part of it.