Hi Steven, welcome to the forum. Worth a search on the SIB forum as there are loads of topics with advice on 'which boat' etc. If towing a Ringo I think a 20HP would be the satisfactory minimum but maybe 15HP if it's a lightweight little one in it.
That really means a 3.8 or 4m with the 20hp and a 3.4+ with the 15hp. Almost all SIB transoms are ply, most simply painted, some ABS covered etc - does not affect the strength or suitability for towing a Ringo - all will be fine.
All comes down to how you plan to launch, trailer or inflate each time? How many people (in the boat and to carry and launch), etc.
Prowave are one of the many Chinese Ribs, similar thoughts here:
WETLINE any good?
I would stick with a well known Euro make (for quality, residual value etc) - there are so many bargains secondhand around at present as per the link above. Check eBay each day. These are a few varied sizes that just came on in the last 24 hrs: