I'll leave current admins like DAR to worry about thread discipline.
My impression is that Shadows I see on sale don't usually stay advertised for more than a month or two, so I'd hope that you can get rid of one if you want to quite easily.
There is a Shadow CD, G-MWVG (known to one and all as George) that was used for ab-initio PPL training from the Shadow Flight Centre at Old Sarum. Looking on G-INFO it is still there, with 4000hrs on it (although I don't think they're regularly teaching on it any more). I passed my microlight GFT on it in 1993! That probably tells you all you need to know about the robustness and suitability for student pilots, if well looked after.
The main thing with the 503 is that it needs regular maintenance - plugs at 25hrs (cost of a set about a tenner), every 50 hours for minor maintenance, monitor crankshaft wear from about 250hrs, and expect to decoke about every 150hrs and rebuild (actually about a 3 day job costing a few hundred pounds in parts, £700 if you're unlucky) about every 450hrs. I'd be very happy to own a 503 engined microlight again any time, having had a couple.
G