You probably want to order the chassis first, from what I hear there can be many weeks of waiting in some cases. That will give you time (but maybe not the money) to get axles etc sorted while you wait for the chassis
My opinion on the chassis is, if you're definitely going down that route then get it ordered!
As depending who you use their is quite a waiting time. I ordered from Richards' and was told a lead time of 10 weeks, I'm now at 14 weeks.
You have the space to store it and it's better to have it and not need it than need it and not have it when the time comes.
I agree with both of these posts. I think I was very lucky, I ordered the chassis one day and picked it up 3 or 4 days later. That was from a local Landy specialist, so they must have had a supplier who had it in stock. Not directly from RC. I wonder if you can put a deposit on it, so you are not paying out the full £1500+ all at once? Leaving you a bit of flexibility with your cash. Or, if the axles are going to take a lot of money, perhaps you could do other work that's more labour than money intensive first. It all needs to be done, maybe in a different order than you first thought.