It doesn't matter how he finances it, he still breaks even on day one.
Day one
Assets - 1 x garage worth £14,000
Liabilities - debt to the bank of £14,000
14 years later
Assets - 1 x garage worth £14,000, cash in hand generated from rent £10,920.
Liabilities - debt to the bank of £5,000 (depends on the terms of the loan of course!)
Of course there are many more variables such as whether the garage is worth £5k, £14k or £100k in 14 years time, inflation factors etc. But ultimately the question on whether this is a sound investment is impossible for any of us to answer. It is relatively low risk with low gains. The absolute upside, as mentioned is if there is a sudden surge of demand for the land due to home building. so the question should really be if the OP can do anything better with that money rather than tie it up in a garage for 14 years (I don't know why we are using 14 years but lets carry on). I would suggest the answer is 100% yes. If you have a lump sum you are looking to tie up and work for you in the long term, it has to go into the stock market. With the compounding effect as well, it is without doubt the most efficient long term investment you can make. £14,000 into a Stocks and Shares ISA, put it in a tracker fund if you want low cost and low risk or pick a fund manager (not Neil Woodford!) if you want higher risk. Sit back and update this post in 14 years with how much it is worth.