The most international class that will have the most participants is Rotax Max (engine) ,,You can be any age and the weight (you and kart combined) is 364 unless your in Masters or juniors..
Here are the international rules for Rotax Max:
http://www.maxchallenge-rotax.com/re.../sporting.html
In truth there are many international classes but This is the most popular and somewhat cost effective... If your looking for something that is less expenseive then you should go into 4 stroke (IE Honda's and Briggs and Stratton)
If anyone has any questions about karting feel free to PM me.. I just finished my 2nd year of karting on a team (GBR) competing in the Western Candian Rotax series .