I would not go the route a GBBR conversion simply because there are now more complete recoil solutions (especially if you're into M4s) like Tippmann and Daytona Gun. However, if you're looking for pure performers then electric solenoid is the way to go. Polarstar Jack is the cheapest, and frankly they're all comparable these days and extremely consistent. Generally it's just a case of slightly more air efficiency or compatibility with a particular gearbox or needing a specific nozzle for more exotic conversions (like the P90 or MP7).
Maybe try watching a couple of videos and seeing one of each in action. The Tippmann will be the easiest to get shooting with as it comes complete and ready to go, but the trade-off for that recoil is that it obviously shoots slightly slower and out of the box they're not as consistent as many AEGs due to their proprietary hop unit and somewhat lacklustre barrel. There are ways to fix that though and a lot of replacements parts are available if you have the cash.
If you're looking for something that just absolutely dominates when it comes to trigger response, consistency and RoF but aren't bothered about recoil then electric solenoid is certainly what you'll want to look at as well. You will have to do the conversion yourself, but it's not a massive hassle and will take around 90 minutes if it's your first disassembly.
PDiK and Daytona Gun both have very specific cases where they win out - the PDiK, for example, is excellent for DMRs but won't go full auto. The Daytona Gun is like a souped-up Tippmann designed from the ground up to provide significant recoil, but it's the most expensive HPA conversion you can do and usually requires specialised tooling.
As far as your 'donor gun' goes - you can really use anything with a somewhat standard gearbox. Just ask if you're not sure, but electric solenoid will go inside basically anything, as will PDiK. The hop unit, barrel and bucking stay the same as any AEG, so you will want to clue yourself up on that as well as your accuracy won't come from HPA alone as it's just the power source. A good barrel, hop unit/bucking combo and decent ammo are the real variables involved in accuracy and range.
Also, when you quote a post you're kinda meant to write underneath/above it or the thread will just fill up with empty posts. You made four posts there when you could have just done one
The 'multiquote' button is good if you're on the desktop version of the site, but if you're on mobile then I don't think there is one.