just to add to what @Lugia said:
get the mantis - just do the simple maths of cost/hp and see what a waste of money the 60hp case + power is compared to the mantis - it's more than double the cost per hp - and you will need those hp sooner or later... it'll also allow you to go for full size modules - a lot of people are surprised by how small eurorack modules are when they actually get them - as they've never seen or touched them before they actually receive their first modules - remember 1u is 1.75" and 1hp is 1/5" - when you start plugging cables into them and try to access the knobs/switches etc - they can be really cramped - and the tiny trimmers that you tend to get on smaller modules are not only really small, but precision is difficult with them...
there's no modulation - you are going to want more than just a random source for modulation - I like Maths as a starter modulation module - see the maths illustrated supplement - it's a great introduction to patching techniques - take the ideas in there and run with them....
there's no where near enough utilities - see my signature - I like a starter set of links, kinks, shades and veils - they cover a lot of bases - which need covering - substitutes are available - Happy Nerding 3*MIA is a good substitute for shades for example - but you will need these modules and more to add variety and complexity
take you time - there's no need to fill a case immediately - but buy or make some blind panels to prevent nasty accidents! nothing dangerous - but they can be seriously annoying if you manage to blow up your power supply and are without your synth until it gets fixed!
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities