First thing is to work out how much money you'll actually have to spend.
On the plus side; allowance, 40ppm for business miles
On the minus side; tax and NI due on allowance, running costs for all miles, insurance (need to have business cover – class 2?), tax, depreciation etc
If you play it right, you'll get a free car – if you do it wrong…
TBH – do nothing and use your current car until you've saved enough to pay for your next car through profit (allowance and business miles), and then continue – eventually you will have a cracking car, paid for.
Also every mile (except home to office and back) is a business mile, even going from the office to a location and then to home.