There's plenty of recommended soils out there, but anything without nutes in it would suffice (I don't use anything special, I just add lots of perlite), I would also suggest not transplanting them into new pots, the stress of transplanting, then transplanting them again into the ground will set you back up to two weeks (usually 1 week of stress and minimal growth after a transplant is normal). Maybe get their ground ready if possible, cut the bottoms off your existing pots and put a trash bag with soil in it under each one until they're ready to plant in the ground. when they are ready to be planted, cut the remaining pot off them and plant them. The less stress the better. heck, having the roots sticking out will stress them less than a double transplant.