There are some key mods to bring in some more of the core challenges of actual space flight (while remaining in hand-controlled hard-mode). Remotetech and a good life support mod to bring some more meaningful differences between manned and unmanned missions while adding some more goals for the ship design and for new missions. Maybe someone can recommend a good life support mod? Secondly, FAR and deadly reentry are both fantastic and again, take more of the training wheels off. Though you'll need to use fairing factory, or fairings from another pack like KW or novapunch to get anything off the ground. As for part packs to give you more options, I sympathise with the pain of people recommending 14 different mods totalling 250 new parts "to get you started" but I would say that to keep it simple: KSPX is a good and not too bloated pack of balanced parts which fit in nicely with the stock stuff, filling in gaps and expanding on a few things but keeping the same appearance and stat balance. Damned Robotics and KAS together add an enormous amount of power to what you can make - hinges, servos, hydraulics, winches, etc. And if you're looking at aircraft, Firespitter has (a vast and endless sea of) aeroplane parts, many of which are pretty redundant but a few are pure genius and PWing is the tiniest but magnificent-est addition to making planes in KSP giving you one completely customisable wing with procedurally generated stats instead of trying to glue 20 or so other wings together like a castaway lashing driftwood into a raft But in short remotetech and life support gives you space flight, FAR and deadly reentry puts it in a real(ish) atmosphere. Other than that, my only advice is to add mods a few at a time and remember you don't need to keep every part, most mods come with far too many!