The best kart for a newbie is one that is bought from and supported by a local shop. The local kart shop is your best resource for questions, parts, and is invaluable to someone starting out! Find out who supports and sells what in your area.
In ascending order of price and power: Shifter engines are Stock Honda (CR125), Mod Moto (125cc modified dirt bike engines) and KZ/ICC (purpose built for karting, mostly Italian made). In relative terms, Stock Honda would be the most affordable to own/operate and easiest to tune.