Matt,
First – there are some excellent G3 wagon kits available from a number of suppliers these days – have a look at Williams Models, Garden Railway Specialists & Walsall Industries for a start but there are others. In terms of low-cost Williams Models have a system called Flexi-Kits. Mike (Williams) also sells individual wagon components – so things like axle guards, brake hangers (laser cut steel) and axle boxes (lost wax cast brass) can save G3 builders a lot of fiddly work.
Second – how much information you need to build wagons in G3 very much depends on the level of detail you want to achieve. Some G3 modellers work to the "six-foot" rule and build fairly simple wagons that are quick to build and inexpensive. Bachmann 'G' (Thomas) wagons can also be simply gauge converted. The kind of detail shown in most railway modelling magazines will probably be sufficient to get the key dimensions right.
Third – The Doug Hewson EIM articles provide a very detailed set of instructions and drawings for a range of 5" wagons. These would be a very inexpensive resource if you can find them or (hint) ask someone to scan them for you!! We do have G3S members who enjoy building wagons in great detail, including all the underframing and fittings. This is very satisfying but perhaps not a first model.
BTW – Paul has mentioned the DH book – Constructing 5" Gauge Wagons – which I have a copy of. It is a boxed set consisting of the 360 page hardcover book itself and sixteen double sided A2 drawings, covering 23 different wagon types – including a 1923 RCH Mineral. The drawings were drawn "half size" (at 17/32" ) – so are bang on G3 scale in theory. However – some simple measurements have shown the printing seems to have caused a few distortions, so a G3 builder would need to be careful and not just take measurements directly from the prints. The 'designs' would also need to be simplified slightly (but not very much) for G3. The book does cost £50 now but for "serious" builders, I still think it would be a good deal – given the total package of plans and book. As an aside, the G3S Editor has asked me to review this book for the Society's Autumn Newsletter.
My advice – for a first effort – either buy a complete kit (or if cost is the over-riding factor) – purchase the key components from Mike Williams (the cast and laser cut running gear parts will save you a great deal of work) and just build a simple underframe and body in 3mm (modellers) ply and/or plasticard. Mike has all the bits required to build a scale RCH wagon – so life is much easier for G3 wagon builders these days… (I'm delighted to say!!)
Hope this helps.
Regards,
IanT
Edited By IanT on 11/07/2017 17:29:48