Really good questions, I'll try and answer them as best I can. Doing it myself was definitely not taken lightly and it wasn't easy by any stretch of the imagination, but at the same time it also didn't end up as difficult as perhaps I'd thought once I got on with it.
My project is technically a renovation/extension so I didn't have any VAT questions to deal with, but I went for self-fit mainly because I got such a huge variation in quotations for the fit. Lowest was about £4.2k, the hightest just under £8k. None included lifting equipment and I knew I'd need lifting equipment even if it was just to get the windows across the front banking to the house. Having personally seen (and read on here about) poor quality installations, I cynically surmised that I'd be better off ensuring detailing was done on installation rather than once the installers had already legged it from site, or further down the road. I know someone who used a known reputable company to supply and install and have had years of headaches down to poor installation, a fight probably going all the way to the courts because every time the company promises to come and fix the problems, they cancel a few days before visiting, or don't turn up. If I did it myself I'd only have myself to blame!
Another reason is that all my ground floor windows are fitted externally to the walls to sit within the new EWI. I needed some additional detailing information about this for the most appropriate approach.
But also to put it into context, I have built the whole house entirely myself, with the only exception of a 2 man team and crane to install the steel frame. I felt I'd be cheating on myself if I got someone else in to do this 😁
I measured, and re-measured, and re-measured, and re-measured.... the as-built rough openings. I know this part is a worry for a lot of people but it's just about being careful with the tape measure and re-checking the readings. I went for 8-10mm clearance.
This really wasn't a problem. I ended up switching supplier having received a final massively inflated quote from the supplier I'd originally selected. This meant a change from aluminium to timber sills and a change in position of the frames within the openings - I had to move the frames slightly out from the timber frame on the first floor. All the relevant technical details about minimum frame support were available from the manufacturer. Then, in terms of weathering, it was just a case of doing the math to specify to total cill extension needed for either the cladding or render portions of the house.
The more difficult process was detailing the rough openings within the timber frame and the EWI as I couldn't find a great amount of easily available and reliable information about this in the public domain. I got some from the window/door manufacturer which was very helpful, but then had to spend a lot of time doing research on this. I made up my own window and door sill trays where necessary too. I also got some detailing information from the EWI manufacturer.
This part of the process was fine but time consuming. I've used a mixture of expanding tapes and airtight tapes internally (on the bay returns and EWI), and Soudal's SWS window system which comprises and external sealant, foam around the frame, and then either an airtight internal sealant or the LQ airtight liquid membrane.
As a note of experience from the inexperienced, I've used a lot of the expanding foam tapes from several different manufacturers in the detailing of the EWI as the EWI requires a lot of extra careful detailing around any openings that comprises several stages of weather protection. What I've found is that when it's the middle of winter and the tape has been in the freezer, you've got loads of time to install the stuff before it expands. In the summer, when it's warm and you're using the expanding foam tapes to fill between about 3 - 12mm, it's expansion rate means you've got to work really fast even if it's been stored in the freezer! In this sense, when working alone, I prefer using the Soudal SWS sealant so you've got all the time you need to get the window into the rough opening, set it all true and plumb, get it fixed and then seal it all up and once set, fill the gap with foam. JMHO.
Another tip is to have a section of soft plastic tube on the end of the foam gun so you can push it full depth between window frame and rough opening to ensure full fill of the foam.
For the smaller windows I made up a simple dolly and used vacuum lifters as hand holds. For the larger items, I made up a trolly using kwikstage scaffolding with wheels (I bought the wheels with the kwikstage scaffolding). On the trolly I made a lifting arm for a chain hoist and vacuum lifter so I could lift the window/door onto the trolly from the pallet, then pushed the trolley round to the lifting point - with the really heavy items like the french doors I towed the trolley with my mini dumper.
For the heay windows I used a rented vacuum lifter hung on the end of the chain hoist but for the smaller units I used a Grabo together with some Faithful heavy duty manual vacuum lifters used as handles to make it easy to move and position the unit.