HO scale is way too small unfortunately. Just right for 20mm figures though. Some parts are still useable but buildings and vehicles are far too small.
I had thought the Airfix Junior/Heller Landrover was too big, closer to 1/35th scale however the two armoured cars are very useful and as they are roughly based on French prototypes a very useful addition to an EU army. They are out of production but seem to crop up fairly often. One model shop in Belfast usually seems to have them and I usually buy all he has when I am in.
Airfix/Heller do a proper kit of a Landrover. It is a much better size than the Junior version but it is quite an old kit of an older model, is of a hardtop (easily fixed) and has terrible, treadles wheels that have to be replaced. Otherwise it is great and cheap buy. I have a couple of technicals but for one the addition of appliqué armour got a bit out of hand and it is now only useable for Mad Max style games. On the other hand it might be just the thing for a Warlord (oddly I seem to be concatenating Warhammer 40K Orks with stereotypical militias – not just African so fortunately it is not racism).
My brother has an ancient kit of a pickup truck that eyeballs out at somewhere close to the right scale. I wonder who it was made by? I suspect Monogram.
As to armament Italeri used to do a very useful set of anti-aircraft weapons that included a number of HMGs and light cannon including a DShk. They were 1/35th scale but work superbly well for technicals. Unfortunately it is out of production but knowing Italeri it will be back sooner or later and then I will buy a dozen or more of them. Other than that the weapon sets by Italeri (a bit pooh), Tamiya and Academy can provide plenty of M2HBs and .30 BMGs and other goodies. Rocket and missile launchers and decent sized recoilless rifles are a bit more scale sensitive and harder to find or more expensive. However they are not too hard to bodge together with styrene tubing and odds and sods.
