I don't know about zwift, but if you want to train on any half decent climbs in bkool, the low resistance trainers are just about useless! A 4% limit will mean almost any circuit will not be realistic. Even on the bkool pro heavier riders find it limiting, but at least it should get above around 10% for an 85 kg rider. Obviously the more expensive trainers, such as the KICKR, will go higher but they are very expensive and will put you at a disadvantage if you compete with other riders.
If you are intending to ride multi-user sessions on bkool, which much improves motivation but may induce you to try harder than you otherwise might, then the bkool pro is the only sensible option, as almost every other user is on the same hardware, so you have a level playing field. We have had riders on cheaper tacx trainers and they have become frustrated by the ease with which they pull away on climbs, and bought a bkool pro in the end.
If you just want to train to get a little fitter, then I guess it doesn't matter much about the steep inclines, but it seems the point of the online simulators is to encourage you to compete against others, or to take on rides that you would like to be able to do, such as alpine climbs and the top 100 climbs, etc. If this is the case, or likely to become the case after you get into it, then I would recommend you get a trainer with adequate resistance levels. Don't believe the marketing, they quote crazy power figures that can only be reached at the highest speeds, at typical climbing speeds they can reach maximum resistance at much lower power figures.
10% at 85 kg seems like a good minimum to me, many climbs exceed this but perhaps only briefly. Only the real beasts get near 15-20%, and the top of the range stuff like the KICKR really give you a good idea of just how hard they are to climb!
P.S. There is a new Wahoo KICKR just out, perhaps there will be second hand ones around? I would say they are much more robust and reliable than the Bkool trainers. I switched from a bkool pro to a KICKR, but did end up regretting it when the bkool riders then started stuffing me in every session!
Geoff