In the original post, you mention looking at websites for (presumably) new kit builds. I have seen both the LSM 4" Foden and the Maxitrak 4" Sentinel. I have to say they are in a totally different league in terms of build quality – The LSM being much better. The Maxitrak has very weak steering and a weird and wonderful reverser lever. The rear suspension has it's parentage from a 750 kg trailer – inventive, but when you really look at it, it's not a model but something that looks a bit like a Sentinel.
As to taking a "lorry" on the road. Register it as a "tractor powered by steam". Legally you are limited to 16 mph, otherwise IVA testings kicks in (which is what kit cars etc. have to go through). Insurance can be readily obtained through Walker Midgley (I use them) or Footman James (Think this has changed names in the past couple of years). See here for actual experience of road registering miniatures:
http://www.tractiontalkforum.com/showthread.php?t=42931
You might need to join TractionTalkForum (it's free) to look at the above. I would recommend joining anyway if your interest is road steam.
I am building a 7" scale Fowler undertype (I used Tony Webster's articles on his 3" scale model as a jumping off point.. My build is currently being covered in EIM (started in September issue) or for the full blow by blow go to:
https://www.flickr.com/photos/140734312@N06/albums/with/72157669955074511
or:
http://www.tractiontalkforum.com/showthread.php?t=20391
Speaking as a designer, I think the undertype is a bit more difficult to adapt to get a driver "in" – probably not an issue if you are working in 3" scale or less.
HTH
Martin