I run a Leopard but have driven both. Have to agree that overall and on average the Rotax is more reliable. The starters suck on both motors.
But it’s still racing and I’ve known Rotax owners who’ve broken down constantly and Leopard owners who never seem to have to do a thing. Anyone who buys a used racing engine banking on it being low maintenance is deluding himself. That being the case, I’d just go with what is most popular at the track you are running at or with the used kart package that gives you the best overall deal.
I wouldn’t worry much about the chassis, so long as there is a current importer. Look at the chassis running in the supernates. Any of those will work and will be plenty competitive at the club level.
Serious advice: Buy a ribtec or other quality rib vest. Do NOT try and save money here by buying something on eBay or from K-1.
Some of the Leopard expenses can be offset with home rebuilds, unlike the sealed Rotax. I’ve done 4 at this point and that motor is about as complex as box of corn flakes. Also, 20 hours (rough Leopard rebuild cycle) is a lot of seat time. Assuming you are not burning up practice miles and are just club racing, it will last you that entire 10 race season.
Tires? My first racing season I ran about 9 races plus occasional practice days and, due to a promise to my wife, did it on two sets of tires. Since I was slow anyway it didn’t make much difference.