There's a web co called the folding sliding door company, they have a clearance section of their shop where they put all their "stuff that was made for a trade show" type gear and some of it comes in pretty cheap considering the normal price of a doorset. You might find at they have one on the shelf that can be made to fit.
http://www.foldingslidingdoors.com/products/shop
For bespoke, I'd recommend white rose windows in Leeds. Unusually for a company found at a trade show, their prices were much keener than competitors and the MD was a good laugh, easy to get on with.
If you do have one made bespoke, by all means give the sizes and get an ETA for delivery, but fit the lintel ASAP. The rest is feasible to knock out the night before but make sure you have a good strategy for ensuring the base is level. The fitters will use packers to take out up to 10mm out of level but they will moan like anything if it's all over the place. Actually they moan if it was being installed onto a concrete base prepared by German perfectionists,but that's what window fitters do
If you bash a hole in the house and have it open for a while, some osb or plywood hoarding will be fine to build a temporary wall out of. A roll of insulation for stuffing in the gaps, or a can of expanding foam would be wide, though the roll could also be stapled to the back of the board meaning your house has a chance of staying warm for 2 weeks, and then I t can be reused elsewher. Personally I'd install the lintel, run the stihl saw cuts under the window and then ask for an afternoon delivery and knock everything out in the morning..
General pointers with bifolds:
Go for aluminium or wood, not PVC
Be very clear what the measurements you give the company actually are i.e. Is it the exact hole size, or is it the size after knocking 10mm off top and sides for tolerance
Give yourself reasonable tolerance all round
You can get away with making it a bit shorter because it's easy to build a lintel down
Measure the width top middle and bottom. Measure the diagonals too because it's no point ordering a 4m wide rectangle if you find when its delivered that the house has a 4m wide parallelogram shaped hole
If your house gets battered by the wind, go for standard thresholds not flush ones - while its a lovely Hollywood notion and architect babble to "seamlessly connec your inside and outside spaces" the flush thresholds are horrifically drafty compared to a proper one
Ask the window co if they will quote for fitting. I had a 4mx2.5m doorset installed, and it cost 10% of the doorset cost (2300 quid) for delivery and fitting. Bolocks is to lifting and aligning something that heavy, for at kind of money. It was a 3 man job for sure, two would be pushing it