Build a 'prepared' chassis. IMOW, GP, SP, CP use basically the same rules for chassis designs. The big difference is the engines. IMOW and GP are going to be the least expensive due to the stock engine rules. Not much need for money for them. SP and CP engines will need some dough put into them to make horsepower. The really good news is that if you build the chassis to the rules, you can hang stock motor on it and run. Save up some bucks and eventually do an engine swap for a new class. Somewhere, in far corner of the deep dark vastness that is HEYMOW is an article written by ROOSTER about how ARMA and USLMRA have very similar class rules. You may search for GEORGE HERRIN's MTD build. (I think it is a 2007????)
Go meet the folks close to you on race day too. We love to show off our stuff, 'specially if it means we get another on the track.
Now, down in the wonderful state of Florida there is a little dirt oval that folks have been running around on for a long time. As a matter of fact it is one of the very first in the country that was strictly built for racing lawn mowers. At that track they have a growing class. GP. They had eleven show up last week! It is growing because it is the most bang for your buck in lawn mower racing. Talk to the fine folks at Florida Lawnracing Association, they'll tell ya all 'bout it.
50 something days left in Afghanistan...... look out Florida I'm comin' home!!