I've no idea why it's crashing when you build walls, fences and stairs but not elsewhere, sorry
You might be able to work around it by using Numenor's AnyGameStarter (somewhere on MTS2). If you set up a separate game for building with fewer EPS, you should be able to build without crashing. You can try various EP combinations for the building game, but I'd suggest leaving Seasons out because it's the worst graphics hog. If you build something you like, package the lot and then install it into your full game.
You could also use the AnyGameStarter to explore what difference leaving out a few of the higher EPs makes. Don't run the trials on your main game and don't forget that you can't take out M&G because your hoods are now in M&G format; they won't load if you take out M&G.
You know that you have to turn all the graphic options down I'm sure; no reflections etc etc.