I dunno, but buying a machine for the purpose of excavating ONE foundation, then trying to sell it seem like a much greater risk than just paying someone to excavate, or renting what is needed to do the job yourself.
Just speaking from personal experience. Built my house 99% myself from the ground up. The other 1 % was friends that happened by at the right time...

So I do have some experience with where the OP is coming from.
Back filling? I did it by hand with a shovel and wheelbarrow... A bx with a loader would have been a blessing. My bet is a BX TLB would suit the OP fine for miscellaneous tasks on 1.5 acres. Could it dig his foundation? Maybe, but it would probably take several months...
As for a slightly larger tractor, whoever said a B series can't move dirt better talk to my tractor. Over the years it has moved hundreds of tons of fill, mostly dumped by trucks though. Digging and moving virgin earth certainly takes a LOT of time with a small tractor. For example, one day I got 16 tri-axle loads of shale to build up my driveway and fill in behind the house next door that I am renovating. That would be over 300 tons easily. At the end of the day it was where I wanted it...
A couple summers ago, I dug and moved several hundred cubic yards of earth to taper the banks around my pond and up near the house. It was a summer project done when the ground was dry, but it was done with my
B2910. Granted, I used my dump trailer pulled by my
BX2200 to move the dirt from the digging spot to where I needed it. But both are small tractors...
In my mind, not knowing exactly what the short and long term needs of the OP really are, if I were where I seem to think he is, I would probably buy a BX with a loader and backhoe. The BX is also a pretty good mower, which I imaging the OP will need over the years.
I bought both my BX and B when I had 3.7 acres, 2 of which were old farmland that was overtaken by nature for 50 or more years, so I had a lot of things to do at the time. Frankly, I doubt something larger than a BX would work for me long term if I only had 1.5 acres. On the other hand, if things changed and I only had the original 1.5 acres that the house I built sits on, I would probably not sell my
B2910, as it has been the hardest worker here over the years and we are pretty good buddies now...