As far as I know, only reason to go 32bit is if processor doesn't support 64 bit.
Yes, most of the benefits only appear after you have 4GB or more of ram but as far as I know, there are no disadvantages. (besides slightly larger disk space usage which is most often not an issue.)
Updating old laptop to windows 10 without official driver support is pretty much touch and go, it could work or it could not.
Probably best course of action is to have working OS on the bottom and upgrade from that, allowing windows to use old drivers if new drivers don't exist.
my old HP4515s updated fine on its own, no extra drivers needed and all works.
Yes, it's from.. 2009 (used to have vista, I had upgraded it to 8.1 though since at 8's release, it cost only $5)
Yes, single core (64 bit sempron, 2.1Ghz)
Yes, 2GB of ram