I don't know what type of foundations you put in where you live, but here in Interior Alaska we need to have foundation footers below the seasonal frost line which is about 42”. Most foundations have a concrete footer with a foundation stem wall on the exterior of the building. If the home is utilizing a crawlspace, just excavate the whole building footprint. This is easily accomplished with a skid steer and is performed fairly quickly. The skid steer machines can move around in tight areas easily and moves an amazing amount of material in a short period of time. I can't be sure of the exact figures and distances, but last year I was excavating and transport material about 150' at a rate of ~30-35 yd³ / hour. This was with my 773 and a 74“ low profile bucket. Sometimes a compact excavator could be faster, but on a large excavations, the compact excavator may not be able to discharge the excavated material out of the work area and it may need to be moved again with a skid steer loader anyway.
If a slab floor is needed, we only excavate for the footers on the exterior walls about 4'-5' and excavate the interior area about 12”-18”. The footing usually only require about a 2'-3' wide excavation so digging with a skidsteer bucket would excavate about 2-3 times the amount necessary. I perfer to perform this type of excavation with a compact excavator or backhoe to reduce the amount of material excavated and the amount of gravel (in my silty/loamy area, 1'-2' of gravel needs to be installed underneath the footer to allow for drainage, higher compaction, and to allow for minute shifting of the building without cracking the foundation) to be installed beneath the footer.
I would say go ahead and take a foundation job or two to see how your machine performs. I think you will profit well and proably get more work from that client. If it doesn't work as well as you anticipate, at least you can never say you didn't try it