The availability of fully pre-machined kits for any kind of live steam engine is a bit limited. The bulk of the cost of production is in the machining of the parts so the difference in price to the customer between kit form and ready-to-run is not as large as you might think. Those customers who lack any form of workshop tend to prefer ready-to-run items anyway and those with a workshop generally machine their own, often from a casting kit.
Beyond a certain price point many people find it more cost-effective to buy a lathe and a milling machine, set up a small workshop, and build their own steam engines or refurbish second-hand ones.
So let's see what you can buy right now, today…
The simplest and cheapest model traction engine kit that you can get is the Wilesco D415. It can be obtained from Forrest Classics – ** WILESCO D415 LINK **
Yes, I know, it's toy steam but they are generally better than the offerings from Mamod and will run at a more realistic speed. Very easy to modify and upgrade with even very limited workshop facilities. Here's one I modified several years ago. There are more pictures in my album.

Also from Forest Classics there are the D.R. Mercer kits – ** D R MERCER LINK **
These are really nice engines and are loads of fun to build though they do require a bit of fettling to get them running well and can be temperamental at times. Very easy to modify and enhance. Like the Wilesco engines, if you plan on running one outdoors a gas burner is desirable.
Maxitrak, Kingscale, and Steam Traction World have also already been mentioned. It is certainly worth telephoning all of them to discuss your requirements. I have always been happy with the service provided by Maxitrak and Maidstone Engineering.
So there you go. Your options for kits are basically Wilesco, Mercer, Maxitrak, and Steam Traction World.
I think it is only fair to point out that for the cost of a 2" scale traction engine kit from Steam Traction World you could fully equip a very nice model engineering workshop. My own small workshop which has two lathes, a milling machine, drill press, bench grinder, and belt sander (all bought new) cost less than a Maxitrak 1" scale Fowler but my needs are modest and I only make small engines and boilers.
For many model engineers wanting to make their own traction engines, making the boiler usually proves to be the most challenging part. Happily there are several very good companies that offer ready made boilers and a boiler making service including Maidstone Engineering, GS Model Supplies, and Blackgates Engineering.
There is always the option of buying a second-hand traction engine and refurbishing it yourself though you may find that you will need to make new parts. I would buy direct from a reputable dealer, never from any of the well-known auction sites, and ONLY if it came with a valid Boiler Certificate. Most of the companies already mentioned have second-hand engines from time to time so it's worth phoning them. You can also try Station Road Steam and Preston Services .
N.B. Anything that is coal fired can ONLY be run outdoors!