Plumbing Question

Hi,
I'm bringing this question here because I've always found people to be patient when explaining things to people like me with more energy than experience. I just bought a new home and I'm starting the life long job of fixing it up. I've got two questions off the top.

1) The tub and sink in the bathroom drain very slowly, but I plan to renovate the bathroom and all its fixtures in the next week or two. Should I try to figure out the slow drain problem now or wait and see if the problem is still there after the new tub and sink are in?

2) In the kitchen and bedroom (directly above the kitchen) There are cast iron drain caps sticking out of the wall. A stack/ vent (one of four, and I'm about to put a new roof on) sticks out of the roof above them, and in the basement below them is the washer and big sink. I'm assuming the kitchen sink used to be against that wall but was moved at some point. I have no idea why there used to be a drain in the bedroom. ANYWAY. If this basement sink and washer is the only thing vented on this stack, would it be better for me to just vent the sink in the basement with a check vent to avoid poking another hole in my brand new roof.

If you got this far, thanks so much and if you have experience with this sort of thing, I'd love to hear your thoughts

The drain lines probably have some build up.. take the opportunity to clear them before/after you demo the bathroom, but before you install the new fixtures. The drain caps sticking out of the wall are the cleanouts for snaking the main stack. How old is the home?

It was built in 1921. There's a lot of cast iron pipe still running across the basement, but it looks like all the supply pipes has been switched over to copper where they turn up to go to the 1st floor.