Hey there ^_^ Long time no see
Nope you cannot trans into anything you want, just the class you were before reborning.
I'd say hunter is a good class to start up, they require very little equipment at low level, you can get to your 80s easily with just what's sold by NPC. Of course having a merch character is always useful to save up zeny with discount/overcharge/vending. TU priest at AM is also a good money maker. The mining system also allows any class to get rich if you're willing to mine a lot (gets really repetitiv however).
Class that level fast in my humble opinion: Hunter, Priest, Assassin, Wizard.
The fishing system allows you to obtain fish, which are items that heal both hp/sp, so you might wanna do the fishing quest early on if you plan to make a class that depends on sp.