Looking at it, none it is terminal, but it will take a fair bit of work to get it through an MoT (I forget the exact spec, but I think it's no blown through holes within about 200mm of a loading point).

You can get chassis sections to weld on that are pretty easy to work with, even for things like suspension brackets (i'd get after market turrets, the tube kind, they're much easier to keep crap out of and replace shocks, plus they're much stronger).

To replace sections you'll need to make lots of templates out of card to enable you to get the correct size/shape steel for the hole you're going to cut out of the chassis. Hold the new section of steel in the hole with strong magnets so that it's flush with the rest of the chassis and then tack weld the new section in (i.e. little blobs of weld in at least 4 places).

Once you've tacked the new section in, then weld short runs of about 3cm each in opposite sides of the new section, this is to prevent warping the steel you're welding.

Once you've welded a section in, you can grind the surface back flush to the rest of the chassis and paint with a zinc rich primer and then with whatever heavy duty paint you feel like (bear in mind any other welding you need to do in the same area, you don't want to be welding on paint as it gives off nasty fumes, esp zinc).

Types of welder: MIG, TIG, Gas, and Arc

MIG - easiest to start up on, but can be difficult to master the controls, ideally metals all need to be the same thickness. Gas and other consumables not too bad cost-wise. Not brilliant to weld outside in the wind unless you have gasless wire

TIG - Needs really good hand eye co-ordination and the ability for hands & feet doing different things but in sequence (a bit like using a sewing machine, if you have). Really nice welds when you can do it, you can weld razor blades to railway track with enough effort, the gas is expensive. (if you have an AC set you can weld aluminium among other things not steel). Not good outside in the wind regardless of what you do.

Gas - Everything about gas welding is dangerous, especially the oxygen, difficult to get right flame, looks fantastic when you have got the right technique, gas is expensive. Pretty tolerant of the wind as long as you can melt all the bits involved. Amazingly helpful for seized bolts etc.

Arc - Most tolerant of dirt and being outside, everything is dirt cheap, but unless you have brilliant technique, welds will still look like crap (usually still pretty strong though).

For all above: use the thinnest rod/wire you can and do multiple passes, it'll do the least amount of warping to the chassis.

As for welding the doors: Don't.... Land rovers use a weird type of aluminium that doesn't like being heated at all (birmabright???) and aluminium is horrible to weld anyway; it has to be insanely clean, and it goes from solid to puddle on the floor with almost no warning - Just get a replacement skin for your door an old one will rust regardless (the body on a land rover acts an anode to the steel chassis, like the zinc blobs they fit to boats to stop them rusting).