Eh, drysuits are the last thing most people buy, after they're sure they love the sport, have no other pressing gear need, and get snooty enough to "need" it.
j/k, there are definitely places where it's almost mandatory, but you won't be there for a while. Save your money.
If your top feels too small, it probably is, especially once you put on some base layers. I would say if it restricts your motion at all, it's too small. Plus, used drytops usually aren't very dry. The NRS Flux for $153 posted above is a good deal; you'll pay $40 for a new neck gasket. It's not the best top but it's 1/3 the price of the best top and has the NRS guarantee. Then, get some neo pants and splash pants, warm base layers including socks (thrift shop is good), some kind of footwear, mittens, a good helmet and skull cap and you'll be set.