I agree on the pedals route for the first time as Critterkllr said!
But if you want to go directly to rack (or api500) units that are more complicated than pedals, I'd say it's not about the difficult circuit but to choose a project that is well documented with available pcbs and maybe even component kits.
Stuffing the board is fairly simple when you have a steady hand and all the components on your desk. But if you do something wrong (99% it's misplacing a component or two) then troubleshooting can be a pain. That is why you need a project with a comprehensive build thread.
Don't go experimenting with hardware - buy a rack case for that particular project. Most popular projects have dedicated cases sold over here by other members! Doing your own is major PITA!
You'll find out that you almost always in the beginning focus on the pcb and stuffing it, but it's sourcing components, wiring, powering and case that mostly take more time than stuffing. Not to mention the calibration procedures. Some are complicated!
Find a mouser/digikey/rs/farnell cart with all required components that someone else already composed and verified and order them at once. Component buying can be a mess if you don't know differences between brands and types.
If I was to pick a beginner compressor project I'd say it's Mnats 1176 or GSSL, as long as you stick to "normal" versions and don't dig into enhancements, mods and addons. You can add those later when you have working unit built.
Don't go anal if your unit doesn't sound exactly like original vintage unit! This is your own version and as long as it sounds good and does the job - it's perfect!
Have fun!
And be prepared to waste your money and not regret it if it all goes into dumpster on your first project that you (don't know how) screwed somewhere!
Luka