What did the heat loss come back at? What size is your HX? The uphill battle to the garage plays no role in pump sizing, as the return trip will cancel out whatever was lost on the way up. With only 80 feet of Logstor, you should be able to push roughly 6-7 GPM with a 15-58 on high or 8-9 with a 26-99 on low. With a 20 degree delta T, those numbers are linear to BTU output (6 GPM = 60k BTU, 9 GPM = 90k BTU). With a worst case scenario DHW call of 4 GPM, you've got 120,000 BTU there alone to get 50 degree groundwater up to 110 degrees which at 6 GPM would hand you a 40 degree delta. If at the bottom of a cycle with 160 degree supply water, that brings your return down to 120- well below condensing temperatures. Add a simultaneous central heat call, you're flirting with disaster. What I would do, is head over to eBay and buy a 26-99 for $150, likely not much more than your supplier was going to charge for the 15-58, and yield the cheap insurance that your boiler will live a healthy life. As for the garage, same rule applies but will depend again on heat load and HX size to determine necessary flow rate.