After a lot of shopping, and talking with my father, who is going in on the gift, and who also called our "local" model train shop (nearly 50 miles away, so "local" isn't quite the right word), I think I'm going to go with the Bachman Spectrum Santa Fe set. I'm a bit uncomfortable with the Bachman brand based on some of what I've read, but I've heard more positives about the Spectrum series than negatives.
It has an oval of track, the engine and six or so cars, and the engine is black and sort of looks like the Polar Express, which will also be a plus for my son. Because it's less expensive than what we were originally looking at, my Dad is insisting on paying for the whole set for Christmas, so my husband and I will buy some extra pieces of track so that Chris can fiddle around a bit with layouts. Unfortunately the local shop didn't have this set, so I'll have to order it on-line; I plan on doing so after the kids go to bed tonight, so if I'm making a major mistake, someone please holler really loud!
I'm also thinking that though I completely trust my son to handle his toys appropriately, I hadn't really thought of the friends-coming-over factor before, so my blood pressure will likely remain a bit lower if his friends aren't playing with a $150 engine. Down the line if he gets some truly fine pieces, he'll have a cheaper one to use with his friends, too.
Again, thanks to everyone. All of the comments have been very helpful, and I really appreciate it. I'm really looking forward to watching his interest and his world grow with this.
Julie