best tip.... dont!
not a goood match in a series, people do it cos they bolt to the siii bell no hasle just a bit of bashing mounts to make them line up, and the motors are cheap & plentiful at the mo, BUT....
power delivery is a bit hard and the siii box already weak, so they can destroy them pretty quick (as can almost any 'alternative' engine tbh, but still)
But, if you do stick one in, the gearing is the next 'problem', SIII ratios being fine for the power the 23.25's have, and giving ratios that will just about squeek them upt to the sort of velocities they have the power to pull, at teh revs they can turn.
TDi dont turn any faster and makes more power lower down, so in the SIII its woe-fully under geared, and they just dont go any faster for the extra power they got, cos they dont have enough gears to put it to use.
So, you start messing around in the transmission area, and look at OD's and rangie diffs and 'stuff' and it all starts getting a bit vexing, by the time you decide that fitting the LT77 'box that was originally on the back of the TDi when hauled out of the disco would be a good way to go to get 25% lift in gearing an OD would offer, from the 5th gear and a stronger box to match the motor.... only THEN the LT230 X-fer doesn't have the same drop set ratios, and without 3.5:1 diffs you loose the benefit of the 5th gear..... While you gain perm four wheel drive, and discovber you ought to have different joints in the front axles.... which after much head scratching brings you back to looking at that scrapped disco agaion and wondering whether you could fit the whole axles.... which you can, with even more hassle, changing steering and chassis and much else, ULTIMATELY concluding with a leaf sprung defender series bodied disco as the end result, a car that is neither fish nor foul, highly contentiouse in the eyes of the licencing authority over original ident and registration rules, construction & use regs and something insurance co's are very unlikely to be happy with if you give them the full story...... cost a lot of money, blood sweat and tears, to get you something that is not 'quite' as good as a stock 90/110/Defender you could have bought for a lot less money and an awful less hassle, IF you even get that far.....
Yeah, they drop in, yeah, people do it.... but what they DONT tell you is where thier little ideas were not 'quite' so great as they hopes, or where the niggles lie.
and when they do, you start to realise that 'sorting' a TDi conversion takes you right into the myriad possibilites of a full on Hybrid build, and hassle after hassle to make each mod to 'fix' the last actually work.
So, best tip I can give, is dont do it.
Look at the alternatives.
If you really WANT a TDi powered coiler, get rid of Series and go buy an old 90/110 to convert with a dead disco engine and 'bits', becouse they fit, basically. Or buy genuine 'Defender', and not have to lift a spanner.
If you like the series, stick with it, live with the limitations it has, or do something more sympathetic.
Plenty of other engine conversions for them, but teh stock motor has a lot going for it, if you give it a chance.
Of the more worthy candidates, 2.5NAD is a worthy choice for an daily useable oil burner with a bit more poke and acceptable ecconomy; looks like it was designed to live in that engine bay (it was!) and doesn't lead to teh hassles the TDI does.....
Elsewhile, as said, google TDi conversion and there will be gazzillions of hits; probably one for every dead dizzy pillaged of its engine........