SS8_Gohan Tenants in common simply means you are on the contract as a partner with your father and therefore all costs/profits are split 50/50 and for tax purposes etc you can claim against the investment. If you where to do it on the side so to speak you couldn't justify the asset for say a personal loan/tax...
As for affording it.... Typically as i said the rental income you receive if you buy in the right location should be suffice to cover your weekly repayments, or close to it anyway, so its really up to you how quick you want to pay off the loan and how much extra money you put in each week..