Non-PAYE income and payment of tax

I'm a PAYE worker taxed at the higher rate. I have the possibility this year to do some freelance work, so additional non-PAYE income. It will only be for this year. It will be about 50k if I take it on and my current understanding is this year I would register to be self-assessed, and then before October 31st next year I make a tax return, revenue then tell me I owe ~25k (approximating 50% of the 50k) and I get them paid to be all square.

I have seen that an underpayment of tax, if under 6k, can be repaid by reducing tax credits for up to 4 years. Assuming my tax bill to be ~25k, is it a non-runner/bad idea/possibility to say put away 10k so I can give a 10k lump sum payment when due, and then opt to pay the remaining 15k by reducing tax credits over a number of years? Would revenue reject that? Or would they impose significant penalties etc.? Basically, I have need for as much of that extra income as possible as soon as possible, but would be happy to repay over a number of years if an option.