Just got my first orders for my start-up - where to go for guidence on paperwork etc? (in the UK)

So I just got my first order for my IT start-up. However it's a reasonably big, complex job that will require me to invest a lot of time and money into it. Obviously, I don't want the buyer to get cold feet or screw me over, so what's the best approach?

Is it okay to ask for some % of payment mid-way through development? Obviously I would inform the buyer of such at the start of the process.