If your on a very tight budget I'd try the Roy wood steam model Janet. The MMS locos won't be any use as they are and by the time you upgrade them cost a lot more.
kGR models in st Mary cray had a second hand unused Jannet at £300 in 45mm worth a call if they still have it. I went down the mamod /MMS route and it's honestly not worth it, they are just toys and the quality isn't great.
If your keen to go the mamod route, tinker, upgrade and pay as you go, I'd recommend getting the body work, frames, rear drag beam and smoke box as spares then buy wheels, coupling rods, cylinders, reversing valve, boiler with regulator and fittings, in cab lubricator, buffer beams and fittings from Roy wood. Dream steam do the parts, and also the upgrades but the quality is not as good.
All these guys are on the web.