Thanks heaps 
Will i have to keep adding rigs as the difficulty gets harder or will this be fine for a fair while yet?
Will i have to keep adding rigs as the difficulty gets harder or will this be fine for a fair while yet?
It depends. If the btc price keeps rising, you will make the same amount of USD over time. If you want to continue to get .28btc per day like you would until the next difficulty, then you would have to add 20-30% more equipment every 14 days.
Sounds expensive doesn't it!