With the build volume I have I can expect more than 5 minutes to put down a layer, so I think the argument about material cooling is moot. With decent settings you're depositing filament onto the existing structure and re-melting it to form the bond.

400x150x150?, material-wise shouldn't be an issue. It's the damned bed that's the issue on that. The CR-10S that Jazz has would fit that (rotated), and that's where you'll often find yourself with an insufficiently-sized printer - trying to rotate the job to fit the available envelope. Or building in sections and bolting together, but only if the design supports that.

There's a strong argument to building your own if you have a specific volume in mind that isn't easily addressed by commercial jobs, but I think a lot of us just want a printer that works out-of-the-box.

One further bit of advice - if you come across resin printers - caution - build volume sucks, curing technology is immature compared to filament deposition, all sorts of problems with flat surfaces on build films, need drain holes for resin, material handling sucks and it does hit your respiratory system. Been there, worn the tee-shirt, gave the printer away.