Have you bought the ballscrews yet and/or decided how to drive the longest axis as I remember discussing it in a previous thread?

Are you going to add some pieces of box section in at 45 degrees at the front and back of the frame to make it stronger and stop it turning into a parallelepiped under high acceleration? I'd also add at least one vertical support in the middle, assuming you're planning on cutting aluminium as otherwise it wont make a tangible difference. Looks like a strong base otherwise.

How are you mounting the 2100mm rails such that they are not bent beyond tolerances? The rails I have for my machine are 2200mm so I've been planning on using self levelling epoxy to get the required accuracy.

There's some good stuff in that eBay link, however the price he says he paid for the spindle implies he got the parts from England not China, so he'll be expecting closer to England prices...

Anyway, to answer the question, you'll struggle to find the longest rails on eBay for a reasonable price, but it shouldn't be difficult to find a good deal on the 400mm rails. Clearly search for worldwide listings. 15mm rails will be fine, just make sure you get ones which have 4 rows of balls not 2.