Ok so Im coming to the conclusion that I will have to build one as Id probably end up rebuilding whatever I can buy.

If i was going to buy one I think the Wanhao I3 plus looks the best as it is 24v and touch screen, that said people still upgrade the extruder and have to brace the gantry to improve stability.

Ive had a dig around and I have all the mechanical bits to make a 500x300x200 machine. x and y belt driven and z on 4mm pitch lead screws. seems a no brainer as then I can get the best electrics to make it work.

Probably go for a 24v system and 32bit controller to get the best resolution

E3d aero extruder and hot end looks good quality and light weight

Any recommendations for the contoller and heat bed?