if you go interest only, as you know you pay the int and dont pay any capital, so although your repayments will be lower the capital borrowed will still have to be paid off in the agreed time so your repayments will be more after the interest only period ends, but not by much as its recalculated over the term of the loan,
for the second, if you think you need to cause it will cost a lot to redecorate then thats up to you but you will be paying it over the whole term of the loan which will cost more in the long run that way, where as if it was for a shorter term you would'nt be paying as much interest