I went with a lot of peoples's suggestions to buy-as-I-went. I have a lot of nice tools now, but I also think I may have saved money in the long run with a kit. I found that it ended up being a self-fulfilling prophecy: Every time I discovered some new moving part to dismantle/fix, I ended up going and buying a new tool just for that tiny project. I also ended up upgrading several tools for which I originally bought a crappy version and then needed to upgrade. When I consider how much I spent on individual (but nice) bike tools, and then look at some of the more comprehensive kits on sale, I think I may have saved a fair chunk of change going with a kit to begin with.

Also, the whole notion you'll buy tools one-at-a-time on sale or at great prices when you need them presupposes a lot of perfect timing: I've found when I actually need a tool I don't have, getting it on sale becomes a several-week process, and I often just end up paying the premium price to have the tool at the time I want it. OR, I end up talking myself into buying a tool I don't really need just because it's on sale. Either way, though, it's kind of defeating that part of the purpose of "buying as I go."

Since you work on cars, you may not need a lot of the tools in a bike-specific kit. There are some kits, I believe, that are smaller but have some of the very essential and bike-specific things in them. Also since you work on cars, it sounds like you'll probably eventually want to work on all parts of the bike yourself. I'd say it depends on how much you really want to work on your bike/bikes in the future. If you're fine just doing the necessary stuff, then perhaps the conventional wisdom as stated by others will serve you fine.

Of course, YMMV.