The problem that you're encountering is that there's no cheap way to cover all of your bases. Choosing one or two oscillators means leaving other possibilities on the shelf. Not good or bad... just different possibilities.
There is no one answer. But there is some advice to be refrained over and over.
Buy a case that's bigger than what you think you will need. Most Eurorack set-ups grow over time. Not having the space for that super-awesome 24HP module you're really itching to get means having to buy additional case hardware for the privilege. Future proof your set-up for at least 1 year.
Filters, LFOs, attenuverters, output modules, and other utilities are often overlooked in most fantasy builds. You want at least one multimode filter, at least one LFO, two envelope generators, four VCAs, and an output module to go with a one or two oscillator set-up.
Don't overlook synth-voices and stand-alone synths that are Eurorack compatible. At the bottom of the pile you'll find Behringer's Neutron. But many other synth manufacturers offer their stand-alone synths and synth voices. If you're just getting into modular synth they may be a much more affordable solution that come with the basics. Besides the Neutron, check out Moog's Mother-32, Grandmother, Pittsburgh Modular's synths, as well as many, many other manufacturers.
Play in the virtual world for free. VCV Rack is a FREE modular synth environment. If you're literally learning the ins and outs of the modular synth world, it's a great way to learn more about different module types. It's not a substitute for modular. But a lot of what you will learn will carry over.