I've seen casette lockrings removed with a screwdriver and hammer but don't recommend this. I tried it myself and could get it to budge, plus the proper tool is like $7 and will last forever. You probably don't need the chain tensioner included in a lot of kits, but a lot of times the kits that come with those will also include multiple cogs which gives the ability to play around with the gearing a bit. You can just chuck the tensioner or throw it up on craigslist or something. I'm thinking specifically of this one from Performance:
http://www.performancebike.com/bikes..._1031512_-1___