That's a big question. Do you want to bend pipe, or tube? tube will have an OD of its named size (ie 1 1/2" tube will be 1 1/2" OD), pipe won't, for instance 3/4" pipe is just over an inch OD.
Also, what do you want to do with it? IME, most pipe fitting jobs can be done with a 90 degree or less bend, so a 3 point hydraulic bender will be fine, if you want to make structures like roll cages or railings you might need more than that which means a different sort of bender.
last but not least, 1mm wall on a 50mm OD will almost certainly need a mandrel bender to get a decent result unless you start filling the tube.
Its worth having a google for toolsforbending.com and having a read of their tube bending manual, which will explain everything you need to know (and more).
And a lot of work to make dies or cash to buy them. Stakesys do a few benders and dies for you to look at or look round google for stuff like blind chicken racing.
If you have a lathe the dies aren't all that difficult to make (unless you need P shaped ones) but the followers are more difficult. I cast mine to the finished curve and they do a pretty decent job. I cast blank dies and machine them to suit the tube diameter and bend radius afterwards.
That's great Pete, I'll have a scan through all that later.
In reality I probably won't go much over 40mm but I thought I'd cover my **** by over speccing it!
The thinner wall will be on the thinner tube so the 40mm will be nearer 3mm thick.
It's for a kite buggy I fancy building so the bends won't be more than 60deg.
I certainly won't be fabricating my own dies.........................yet!
Also how snug do the dies have to be? For example would a 3/4" but suitable for 20mm tube??
no, the dies need to be a good fit to the tube. Ideally you want a U shaped groove more than half the tube diameter deep, and the follower makes up the rest of the circle (or most of it)
Up to 11/4" or so its not that hard to bend mild steel tube. A hilmor type plumbers bender will do it with the right dies and an extended handle. Mine has bent 1.25" by 2.4 wall to well over 90 degrees without any issues. I'll try to remember to stick some pics up when I get home.
I'd of thought Buggys would be made from light weight tube, around 19-30dia and thin wall:1.2-1.6mm. Otherwise it will weigh a ton, and take ages to accelerate.