Not used TV tuner cards so can't comment on that. I wouldn't use the aerial lead anyway, all that RF conversion at either end makes for messy pictures. Never managed to get the scart -> s-video input working on my VC100. I get a picture, but it's in B&W.
I have successfully used the composite input to grab some stuff in colour from video. The results are OK, but don't expect DVD like results. The capturing of VHS tape often brings out the imperfections that may not be visible on a TV and you may need to tweak the video until you're happy.
In answer to your questions, 1 Scart -> Composite ? No chance of it going bang. Go for it.
Hopefully that should get you started. I can recommend VirtualDub MPeg2 for editing your captured files & it has powerful filters that can be used to greatly improve the image quality... The mega codec pack has just about every codec you can think of.. but you've probably got software that came with the card & it depends on what you're planning on doing with the video afterward of course.
Paul Clarke's blog - I live in Hythe in Kent. Wed + dad to two, I'm a full-stack web engineer, + I do js / Node, some ruby, other languages etc. I like pubbing, parkrun, eating, home automation and other diy jiggery-pokery, history, genealogy, Television, squirrels, pirates, lego, and TIME TRAVEL.