MKV 2dr Rabbit AHU swap

Hey guys first post here, done a lot of lurking here (lots of general VW info around here) and decided to register for this one.
I have a 2007 VW rabbit 2 door, 2.5 I5, 5 speed. It's got 175k and the other day it started leaking antifreeze out of the coolant temp sensor, prompting it to over heat, and then the next day my crank bearing came to a screeching, smoking, oil boiling stop(I pushed it trying to get the car home).
So I got to looking around and couldn't find any cars with running 2.5s under $1000. Call me crazy but I ain't paying $1000 for a used engine. But what I did find is several MK3 Jetta and golfs with AHU engines that run, the body's are just rusted and aging. I can pick one up for $500 or less for the whole running driving car. I love turbo diesel's and have always liked TDI's so it seemed like a good option.
I know you guys are going to say just skip the BS and go for the ALH with the VNT turbo. But they are average $800 for an engine only in my area and for me the AHU is skipping the BS, there's no immobilizer, very basic engine harness, so basically it will be the easiest to drop in my bunny and get it going, no computer tuning should be needed.
I have a few questions, and as far as I have found no one has done exactly what I plan to do, probably with good reason.
1. ECU- seems to me I should be able to run the ecu from the engine donor car to control the engine, but will I need to keep my factory ecu in my rabbit to control other things?
2. Transmission- from what I have read the AHU block should bolt to my 5 speed transmission in my car, is that correct?
3. Engine Mounts- I have read that MK3 and MKIV mounts are interchangeable, any such luck with the MKV or did they change the design?
4. Fuel Tank- I have my doubts that the MK3 tank out of the donor car will fit my MKV, does anybody know if the pump will fit in my tank?
5. What is involved in having immobilizer deleted for a ALH ECU and what's the average cost? At this point that's my biggest hold up on the ALH, because I was rather have one in my car but it seems like it would cost twice as much.
Thanks in advance any input is appreciated thank you!

In the USA (and Canada) it is illegal to put motor older than the car is. For many, it will never be checked, but something to take into consideration.

Also your MK5 and the AHU and even ALH motors won't be a good match for your MK5 - the reason is the electronics. If you don't mind the rest of your car panicing that it can't communicate with the ECU then it will be fine... tachometer, water temp everything engine related in your instrument pod for example won't work as the AHU computer doesn't speak CAN.

Engine mounts are different too - the ALH would be a better choice.

Honestly, this won't be a good swap for you to do!

Kind of reminds me of the 1.6TD I saw installed into a Mk4 I saw years ago. (Long before we had etest here on Ontario)

I had full expectations of non working gauges and all that fun stuff. I have been doing more reading and decided that the ALH is a better option all around, and I can get the immobilizer deleted for $60 or less. Would the ALH ecu be more friendly with my car? Are the MK4 and MK5 mounts the same? I really want a TDI in this car. I know there was no MKV TDI golf/rabbit in North America, but there was TDI Jettas, seems those are the BRM pumpe duse engines. Would that be my easiest drop in swap? Because I don't know how much I like the idea of one of those. For one you can't EVRY mod them which was something I was going to do while I put the car together. Also ive read that the cam lobes eat them self's every 150k or so. I have a heavy foot and a love for boost, I need something stout. I mean, after all what's the point of a diesel if it won't take a whipping?

BRM is your best bet for compatibility.

BRM would be a much better (and easier) swap. I do not think the AHU into that new of a car would be worth the headache. Not to mention it technically is not "legal".

Good news is, replacement 2.5L engines are plentiful. I'd find a 2009+ one as they have better improved internals, you just won't use the vario oil pressure system.

So the BRM I assume uses the same mounts as my MKV 2.5? Would the ECU from the BRM work with my cars electronics with just a immo delete?

I've been keeping my eyes out for a cheap 2.5. Will any year 2.5 work with my ecu, mounts, and transmission? I saw a 2012 engine for $200 said it ran good but it was already out of the car and I don't like the idea of that could be toast and unless you start tearing it apart before you buy it you'd have no idea.