Crits aren't that bad unless you get people who don't know or don't care what they're doing. If you're really worried go out to your local crits and get an idea of how the lower grades in your area behave on the bike before joining in. Also see if they allow "self seeding" or if you're allocated a Category. People getting to choose themselves can be an issue - just because you sat in the bunch all race and sprinted to 5th a few weeks in a row doesn't mean you need to go up a category.
If the commissaire has to lay down the law beyond the usual "roll through when called, sprint straight, hold your line in the corners" maybe look at another club's races. If you know anyone who races at a club you're interested in ask what the behaviour is like and how often crashes happen.
Do some shorter, sharper bunch rides with people you trust and get used to following wheels through corners without swinging wide, not overlapping wheels, rolling through, closing gaps etc as well.
As for gear, unless you're serious in higher categories it's better to not race anything you aren't prepared to replace. That Cat 4 on the S Works Tarmac SL7 might look cool, but if he comes down hard in the bunch that's some money down the drain. There's a reason Allez Sprints and CAADs are popular
Don't over think it too much, IMO racing crits improves every aspect of your riding except climbing, and they're great fun when you're in the right grade.