Your payments are not deductible under any circumstances. You enter the garage as an asset and report the total price you paid for it. If it's for your personal use only, then you'll indicate zero percent business use and nothing is depreciated. Otherwise, indicate the proper percentage of business use and it gets depreciated over 27.5 years. The amount of depreciation will depend on the percentage of business use of course.
I'm questioning also what type of loan you have. Is it a mortgage or HELOC? If so, then claiming mortgage interest shouldn't be an issue, as if you enter it correctly in the program on that portion of the interest that applies to the rental part would be deducted on SCH E, with the rest going on SCH A. Then for property taxes, the same "split" will apply. More than likely your property taxes won't include the new structure anyway, until your county tax appraiser has assessed it's tax value.
"do I enter in the full amount of the kit, or just what I've paid in payments so far?"
You won't enter anything on y our 2018 return, since the garage doesn't exist as of yet and did not exist in 2018. But for your 2019 taxes your cost basis on the garage will probably be more than what you paid for the kit. Your cost basis will be what it cost you from the time you said "I'm gonna buy that" up to the day it's placed in service. So if you had to pay shipping delivery charges, that's included in the cost basis. If you pay for it's assembly/construction that cost is included in the cost basis. If you have to pay for surveys, construction permits, licenses and the such, that too is included in the cost basis. Just make sure you keep all your paperwork together so that if ever audited you can "prove" your cost basis.
