OK so depends lol.
Can spray is fine for priming, but you are going to need some where with decent ventilation, so take it to the car park, roof top or grass space the apartment might have.
Air brush, really depends on skill level, but even at a novice level, useful for quickly doing the base coat of a lot of infantry or vehicles. If you are just doing that, then a cheap airbrush with a decent compressor is all you need.
Nothing wrong with using a brush, just takes longer.
Basing is a massive topic. Common things like bark repainted as large rocks works, so does cork and since it can be crumble can be used for smaller stones of generic rubble.
Sand is often used, but has scaling issues, baking soda is another option as it is finer.
There is also the "pro" flocking, grasses, snow, ash, they all give a nice effect. If you are looking to do short grass effect, I recommend mixing a few, grass fields are not uniform in colour.
The off he shelf long grass or tuff can also be a nice touch.
Then there are custom bases, heaps of those.
Textured paints are over priced, really just mix normal paint with grit/sand/whatever and go nuts. Cracking paint is nice for Ad Mech though.
Best advise for starting out with basing is to find something you like the look of, the general idea is that it helps the model stand out. Then post "hey I like this base any idea's on how to do it"
At the end of the day its a bit of trial and error to find something you like.