In 2011 there will be plenty of events in michigan and the surrounding areas. MDU, J woo (check Project A. Spec), and I think others will be hosting events in michigan. MDU, Clubfr, and outbrake motorsports also will be hosting tons of events around the region so be sure to check them out. Event dates and locations should be posted over the next month or so and I'm sure there will be a thread on here with all the information on them.
Welded diffs work just fine, great if your on a budget. If not I'd recommend a 2-way clutch type lsd over a torsen for drifting.
As far as tires go junk yard stuff and used tires should be fine till you really start getting into some power. I'm pretty sure stance makes coilovers for the miata and I'd highly recommend getting a set of those as they are great for the money.
Look forward to seeing you at the track!