A bit less DIY but similar functionally is the kit Shades of Coffee sells that also adds PID to the Gaggia classic.
If you're just after the functionality of a cheap espresso machine with PID that takes a lot less effort.
https://www.auberins.com/index.php?main_page=index&cPath=6&s...
But the recommendation was more for budget purchases. Breville also do a dual boiler with 58mm which is more expensive but if you live in Australia is amazingly good value (often $900AU which is about $600US). If you have more money still get a La Marzocco or whatever :-)
My taste is to avoid fiddly things that need too much skill like heat exchangers etc.