For the Andes class, I think you're going to have real trouble starting with the connie 2-8-0, which is not only NG outline, but outside framed. I'd be more inclined to start with one of the Aristo eight coupled locos - either take the new 2-8-0 (still pricey), reletter it and then do more fundamental body conversions to bring it closer to the Andes as time/skills/budget permit, or buy a second hand 2-8-2 (probably cheaper) and perform some more immediate surgery to change the boiler for a parallel one and shorten the back end before putting it into service as a 2-8-0.
Diesels are probably easier/harder, depending on the prototype accuracy you want (assuming here you're working in 1/29). None of them are available RTR, and with the possible exception of the C30-7, aren't likely to be either. For accurate reproduction, probably the only option is to source suitable wheelbase trucks and set to with the plastic to build the body from plans. More sketchy versions could be made by conversions
Alco D560D - reckon the nearest you'll find is the LGB white pass diesel. This is supposed to represent an Alco/MLW DL535 NG loco, but this model in particular received liberal doses of LGB's 'rubber ruler' so you might find it actually looks all right. Worth borrowing someone else's to check for size with your other stock before commiting I'd suggest.
GE C30-7 - a suitably massive rebuild of an Aristo dash-9 ought to be possible - the trucks at least are basically right, and you'll get a lot closer appearance wise by changing the cab and the radiators, but for real accuracy it might be preferable to build a new body. At least one person has built a C30-7 (
http://www.mylargescale.com/Community/Forums/tabid/56/aff/8/aft/123680/afv/topic/Default < Link To http://www.mylargescale.c...afv/topic/Default - scan down to the fourth post ), but I don't know how much is new and how much is donor aristo.
GE B39 - Should be able to get quite close with a simpler Aristo U25B rebuild - bulk up the nose a bit, modify the cab roof to the more modern (flatter) pattern, as well as raising the cab generally, change the skyline on top of the long hood to mimic the B39, and I think it'll look pretty close. For once the B-type trucks are wrong, but the sideframes are cosmetic and just unbolt, so you could leave them in place until you were happy with the replacements.
Jonathan