2 piece is easier to wear, allows taking top off while wearing at the race but not in the car, cooler then with top off since Nomex makes you hot when its on.
Proban is cheap and does not last long and you have to replace
A single layer Nomex suit costs less then a multi layed model but some race groups will insist if its single layer then you also need to wear Nomex tee shirt and underwear where its not needed if suit is multi layered Nomex.
Also if race rules require you need Nomex, shoes, socks, gloves and Nomex lined helmet
Nomex gloves can be thick thus fingers can be harder to pick things up with them on. Get ones that fit good.
I suggest you get used to wearing all the gear including the helmet which can stress neck, also in doing this you get used to the extra heat all this gear causes to the body.
I use to wear mine for hours at home just to break the new gear in and get used to being in it.