There are weld-in replacement parts available for series bukheads, including door posts and feet. Installation requires cutting and welding so may be a bit tricky for the novice, but not a tough job, all in all. Chances are, if the doorposts bottoms are gone, the bottom corners where the windscreen meets the bulkhead, adjacent to the windscreen hinges, is probably also going too. There are parts available to fix that too. Go to the Rovers North online catalog to see what the pieces look like.
I've just done a chassis-up rebuild on my series II and had to do that on both sides. I got them from Autopost in south london (advert LRO,etc.internet) They were really cheap considering what they do.
Stating the obvious, one side at a time.Take your time and take as many measurements as you can making sure they marry up before you weld inplace. eg from out-riggers and chassis rail,etc. Make a mark on the nearest good bit of bulkhead post and take that down to the centre of the fixing bolt hole. I managed to get it only 3/8" out and pulled that back with an hydrolic clamp (hired). I've fitted the side rails and the doors back on now and they are fine. Go on give it a go.
Good luck, you won't need it!
Please don't take offence at this, as it's meant constructively and not in insult, but it sounds as though your current level of expertise is not yet enough to undertake this job. It's fiddly, cumbersome, time consuming, intricate and requires precision. It also requires accurate cutting, jigging and quality welding (which I certainly am no good at). My bulkhead would be comparitively easy to repair, but I'm replacing it as I wouldn't trust my wormanship. I'd suggest trying to get a friend or fellow enthusiast to do it, or failing that, a decent garage.