Hi Heinzy.Sounds like a great idea.
As mentioned before the only plastic kits Im aware of are the 2 Trumpeter german trains from WW2 in 1/35,and I seem to remember Airfix did a couple of really small kits about 40 years ago!
Your other choice is etched brass/white metal construction in either gauge 1 (also know as "live steam" and huge amounts of money!) or 'O' gauge which is about 17"-18" long for a typical loco which is a nice size.
You dont need to make them motorised,as they dont come with motors couplings or wheels (some dont even have sprung buffers included)and I would think you could superglue a lot of the loco together if its only for display IF it has a cast metal boiler and not a brass one,as this would have to be rolled and soldered I would think.
You usually buy the kit,then go to Slaters plasticard and buy the wheels and couplings from them so dont forget to add this to the cost too (although DJH do offer these as options to buy on their website).
Kits....theres DJH as mentioned,expensive,not the most detailed kits out there but easy to build (and they do fully built and painted ones if youve deep enough pockets).They could be superglued together as the boilers are cast in one big lump so no having to roll the sections of brass boiler and soldering them together.
Then theres David Andrews kits,which are really nice to build,but soldering is a must as is rolling the boiler!But they are the cheapest out there I think.
http://www.locomotivekits.com/The top of the line is ofcourse Martin Finney kits which are incredibly detailed,but again soldering and rolling the boiler are needed.But they are beautiful kits if not easy to make.
http://website.lineone.net/~cbwesson/7mm.htmTheres also Piercy kits which are hyper detailed (see DJH website),but I think it MIGHT be possible to build an A4 (eg mallard) with superglue only as again its a cast metal boiler.But you may have to reinforce the joins of the tender sides,and the cab would have to be soldered up as I cant see another way of doing that,but everything else could be glued thinking about it.
As the running gear is all bolt together so no solerdering there,as as its static I dont see a problem.
But dont let soldering put you off any kit,as its easy to do,theres lots of tips and guides online for soldering locos up and what tools etc are needed.
But as the previous poster said,give Tower Hobbies a phone call and they will help you out,as customer service really is fantastic,service top notch and theres nothing they dont know when it comes to O guage locos.
Jon