The wagon enthusiast is well provided for by the model trade in O Gauge.
Early model wagons were generally made of wood with cast whitemetal underframes. As time progressed and photo etching was found to be reasonably inexpensive, a range of wagon kits were created by people interested in selling such kits to other modellers. As numbers of O gauge modellers increased, injection moulded wagon kits became available and inexpensive. The level of detail available now is very high. Now the range of wagon kits available is extensive and covers all the major railways in the UK.
Latterly, ready to run wagons have started to become popular. These are injection moulded with metal tyred wheels and some include compensation to keep all 4 wheels on the track.
As models and kits were released, the guild magazine, the Gazette, has published various reviews of wagons and kits available for purchase.
Some guidance in rolling stock construction is available as part of the guild manual.
The essence of this piece by Tommy Day appeared in the November, 2020 Gazette in Volume 21 No 5, p79 (http://www.gaugeoguild.com/secured/gazette_archive/Vol21-5//offline/download.pdf#page=79)
Buffers/buffing gear can be solid or sprung on models but have always been sprung on the rail network, except in very early days. The simplest model form is solid buffers. This is fine if the couplings between vehicles hold them apart, but if the couplings are flexible and the buffers are to serve their actual purpose, especially when the train is being pushed, then some form of buffer springing is preferred.
I had available a couple of basic wagons with no buffers fitted, so decided to try some common springing arrangements for comparison. There are many self-contained sprung buffers commercially available, and many bespoke methods of springing buffers. The following photos show a few examples that I’ve tried.