Maxitrak here in UK MIGHT be a good place to start, although I have to say that their electric locos are based on British NG rather than US practice. There are numerous companies in the US who will build just about anything you care to name to run of this gauge track - from a DD40-X down to an NW2.
Here are a few prices from one manufacturer -
SW-7 $6494.25 (gas/hydraulic only)
GP-9 High hood $11,497.50 Gas or Electric $13123
GP-9 Low nose $12,022.50 Gas or Electric $13374
GP-38 $11,546.85 Gas or Electric $12450
SD-9 $12,757.50 Gas or Electric $14334
SD-45 $13,275.75 Gas or Electric $14144
SD-40-2 $13,675 Gas or Electric $13429
C-415 $11,250 Electric $13162
C-420 $11,597 Gas or Electric $13263
RS-1 $12,425 $11,151 Electric $12,651
RS-3 $11,745 $11,278 Electric $12,778
FA-1 $11,975 Gas or Electric $12,756
PA-1 $14,995 Gas or Electric $15,623
Dash 840CW $12,875 Gas or Electric $13,768
Dash 840 C $12,275 Gas or Electric $13,190
Dash 9 $13,484 Gas or Electric $15,176
AC 4400 $12,897 Gas or Electric $14,750
Trainmaster $15,950 Gas or Electric $16,500
E-2 $23,453 Gas or Electric $24,578
Engineer cars.... Half Gondola .. $1,475 Flat with low seat.. $1,475
6 foot steel gondola w/trucks and couplers... $985
"Track Snapper".......$135 (includes ground shipping)
If you intend to run NG prototypes on this track, then capacious pockets will be needed - live steam NG on this size track is humungous - a well-built K-27 will set you back around $100K.....plus the cars.
You really need to have a better idea on what they have on their minds ere you go any further. Knowing what the budget for track and locos is vital information that you don't have right now.
The post above this one has a great link, BTW, much of what you want to know can be found there.
In this scale, NOTHING can be done on the cheap - health and safety issues are paramount, especially where the public has access, so braking cars to the appropriate standard is required for a start. Have a look at the Train Mountain site to see what we mean.
Secretary - RMSRS [www.fenlandlightrailway.co.uk]