I presume I could buy most of the bits from ebay and cobble it together with relatively few home made parts ?
You can, and you do get some use from the cheaper stuff, they just wont stay accurate for long, without adjustment.

What is your realistic budget for the total project, and what space do you have for the machine. what size do you want

Then, can you wield or know a mate that can.

if you can answer them, you will get some help within the budget your looking at. For £ 1000 to £ 1500, if you can get the welding done cheep, you can build a reasonable quality starter machine. ( 300 spindle+VDF+fittings, 400 to 500 motors,drivers,card £ 300 - 450 rails+barings screws+fixings ). 100*better and more reliable than the one you pointed to.

You could do it cheaper than that, or get better quality parts for more, but that gives you a general idea of what it costs to get a decent quality home machine.