Look to the Stuart site. Look at the price of the casting sets, and then realise that Stuart's do sell *some* of their models ready to run, at six times the casting set price. For many people on this list, the joy of building castings into engines is what it's all about... To gauge real collector interest, just put each one of yours up on ebay individually, with a silly reserve price- if someone wants to buy, sell! If not, you'll have an insight into the current market... You don't have to sell through eBay, but you can use them for research, for free. Easy.
Also bear in mind that finished engines that Stuart's don't sell themselves should command a bit of a premium, if nicely built and finished.
You'd best be aware that if you don't clean and repaint them, someone will probably buy in order to do just that, then turn them around again. Best return for a bit of elbow grease and two pots of paint will come from the beam, and the no.4 vertical engine. The latter can actually be regarded as a small working engine, in fact.
Incidentally, there has been a Stuart centrifugal pump offered; it was designed to couple with their "sirius" engine. The castings have been going in and out of production for years, as has the model. The one you have just might be one of those.
-Andrew UK