Maybe start with a Syntakt, as it is a streamlined “best of Elektron” with enough tracks to make a whole set/song on. It also gives you a little bit of analog and FM synthesis in a streamlined way. Lots of input on this thread:
You will get to know the Elektron workflow and see if it’s for you. If you then find you miss anything, add another digi from there. Say you notice you need samples, go for Digitakt. If you notice you would want deeper synth sound design and/or polyphony for e.g. pads, add a Digitone. Starting out with all three of them would be mental overload and a waste. Most of all: have fun, you’re in for a treat!