...is not the machine that the heavier VMC40 and 40-20's are. So I would say the 15 XT is not quite the machine the box way machines are. They have lower HP and are a lighter framed (weldament) machine. Don't get me wrong, I think they are good machines and fairly easy to fix or repair. I base this on my personal history with both machines keeping in mind that the 15's and the other smaller machines are there to fill the lower price and model shop market.
What price range are you looking for? Any used machine is going to need some repair. Unless you get lucky and find one that has low hours (hard to know the hours used for sure, most of the time it is heresay) and was out of a shop that had a good maintance program. ( again heresay).
I think it is how much it is worth it to you to pay and maintain the large machine. Larger is almost always better with these machines. I will say that I personally think the best Fadals made are the 1989's and into the early 90's VMC 40's (I think they were later called the 30x16 box way machines) and the 40 x 20's. Oil mist lub and 15HP 10k. There are alot of them still out there, chuggen along, all day, every day.