There is no getting away from it, fitting a new base is not easy. Did yours come with slots cut in it? I've just done a Jennings which did, but I can't remember if the Mills ones are slotted. Even if it is, it may well not quite fit, which is a real pain, and then added to that, fitting a base means a complete strip down of the case, top and bottom castings, pump, and main plate. The main plate is important - mark its position where it meets the sides, so it can go back into the same position when you rebuild. Is is important for positioning of the mech with regard to the JP etc. The main plate might slide out backwards, but usually not, and needs a bit of "jiggery pokery" to get it out and back in again. You may well need to drill holes in the new base to match the side connections. You will also need to position the pump with care, but this is a good time to service the pump (it has no oil in it, only on the shaft).
I differ from the hard purists, I like to restore machines once they have passed a certain stage of distress. I think yours has. I would strip and repaint in an original colour. This is my view only and the gurus may well disagree. I think they look better and get a re-birth.
How many coins do you need...? Loads../ think of a number and double it. The jackpot alone, if set for max size will take 200. The coin tube? Another 150.
I've never seen anyone mention this, but a stack of 10 shillings worth of 6d coins (20) is exactly 1 inch tall, so you can measure it out
Come to Coventry, there are usually loads of 6d for sale and, dare I say it, loads of spares at half the price HP charges.
Here is a flyer for the type 'D':