Recommnendations: Get a bigger rack. It blows having to spend a bunch of money on power just to turn on other expensive stuff, but it’s how it goes. You can replace it later, but if this is an investment towards growing into modular, you’re giving yourself a hard handicap very quickly.
Get the modules that make you the most excited, but don’t forget utilities.
Maths is popular as a first module because it’s super versitle. I didn’t go that route, and occasionally I regret it. It gives you two loopable envelopes and a bunch of attenuverters with logic in one package. It’s crazy popular for a reason.
If you want to be a bit more hands off, Marbles is a lot of fun and very versitle, but it’s not going to teach you as much because it’s doing things for you. It will give you lots of triggers, CV modulation, and note CV if you want it to experiment.
Morphagene is a very rewarding module. It got it early, and eventually traded it, but not because I didn’t like it. It’s just a lot of fun to play with. I’d pick it over Clouds every time.
Stages is under-appreciated. I’m not crazy about the envelopes it makes, but it does a ton in a small package. You can learn a lot just from self patching it. It can give you syncable LFO’s, oscilators, sample and hold, sequencing, offsets, any envelope type you can imagine, and a secret mode that give you a harmonic oscillator. I have actively tried to make sense of replacing it, and I can’t seem to do it.
Rosie was my first output module. No regrets there. Crossfadable mono inputs and a stereo input/effects loop with headphones and line outs. Very very useful.
You’re on lines, so I’d be remiss not to recommend Three Sisters. It’s basically a mixer filter hybrid and it’s crazy useful and sounds crazy good.
Ripples is a solid choice though. It’s a very good filter design with a built in VCA. Good stuff.
I like ILFJ, and I have it racked, but @alanza is right that you could just as well have it in pedal form. Though that gets to the question about whether you want it pedals at all.
Effects in and out of rack is a big question that people debate about a lot. Don’t sweat it too much.
I also recommend getting a few stackable cables early. They’re really handy for experimenting. You’ll want more cables than you have faster than you’d think.
Buy what excites you.
Don’t forget utilities.
You can always sell or trade stuff later. Most bits of eurorack kit resale well.