Hi Rus
Disco's will suit your needs nicely to get into the sport. Surprisingly capable and as you say, cheap to buy. They are also easy and cheap to maintain.
The best bit of advice I can give you is get one, drive it, compete in it, talk to the other competitors to see what they are doing and first and foremost, enjoy!!
The basic mods to do before competing will be a simple bit of protection. Diff guards, and either a steering guard or uprated steering bars.
Add to this some solid front and rear recovery points and get rid of the tow bar.