I bought just about everything that I purchased used, either from thrift stores (i.e. Goodwill) or at Once Upon a Child. And received a lot of hand-me-downs, which was fantastic.
The gear thing is frustrating because you just don’t know. I always meant to try the sling, but couldn’t figure out where to buy one (I think they’re more widely available now). I also found out that I’m the kind of mom who needs to get out of the house and go places with the kids, so I needed lightweight gear.
Diaper bag - I went through 3 different diaper bags, the first one wore out and I tried two others that didn’t quite get it done, finally wound up using my old backpack.
Crib - I co-slept until they were about 8 months old (and occasionally in the years since), but the cribs were great for naps, particularly once they started rolling over. We only used them until the twins turned 2, when the movers lost some parts. We went to plain mattresses at that point (they’d started climbing out at about 17 months or so).
Changing table - great for the first year b/c of doing 7-8 changes/day/kid; ditched it when we moved, once they started running around it was easier to wrestle them to the floor & pin them under my leg (I can’t remember anyone telling me ahead of time about that phase
).
Rocking chairs - still love them. Ours are regular La-Z-Boy recliner/rockers (from about 1972) that a friend had in her basement. The armrests are really low, and padded. Very helpful when cuddling a sick child.
Stroller - I wound up liking my lightweight umbrella stroller a lot, but the big DuoGlider was useful when mine were babies, it had more support and hauled stuff. Both of them came to us used and were still donatable when we finished. We still wind up carrying our kids sometimes, but strongly prefer that they walk.
Car seats - Had the Graco bucket seats for the first 10 months and loved them, my babies often fell asleep in the car and it was so easy to get them inside. Then we went to the 5-pt Graco for about 15 months, and now we have the ones that are combo booster seats (which should last for a few years).
Swing - one kid liked it a little, other one hated it a lot. Same with those doorway jumpers.
Exersaucer - there’s an older one by Graco that my kids loved to bits, it didn’t have many toys attached, but the 3 legs were springy. Really great from 4 months - 10 months, then gone.
Pack-n-Play - limited usefulness for us, our kids didn’t tolerate them well; OTOH, when we needed them for safety or convenience (or traveling), they were very handy. Useless after 17 months, they’d just climb right out.
Play pads with the toys dangling from arcs - fantastic from about 8 weeks to about 8 months, then useless.
High chairs - won’t need at all until your son can sit up (6 months?). We started out with conventional seats that I bought used for like $12/ea, but they were so honkin huge. So I switched to the plastic ones that strap onto your chairs. Loved them.