1) Use whatever equivalent you have to our 'Poundland' They are a great source of:-
Acrylic paint sets; possible sci-fi vehicles & figures. (e.g. replace the head of a WW2 figure with a mapping pin
for a quick & cheap sci-fi figure) also possible scenery (e.g. beads are great glued to a base as containers. Water
pistols with triggers removed & with a few 'bits' glued on, make great sci-fi machinery)
2) For beginners I have found that they get reasonable results with dry brushing.
Stage one = Undercoat (I use 'match-pots' as they are cheap.) For most historical figures I start with a dark brown,
though for sci-fi it could be just about any colour. Leave it to dry (overnight if possible).
Stage two = Dry brush a medium tone, again I use match-pots (over a dark brown undercoat I use a 'coffee' brown)
Stage three = Dry brush a 'highlight' (to continue with the above example, I use a light sand colour)
Stage four = Paint a little, a VERY little detail e.g. weapons, visors, backpacks or flesh (for sci-fi flesh can be just about
any colour. For the next stage to be most effective, stage four is best done with light colours.
Stage five = When all the above have dried, a wash of ink (Peat brown for me, but great results for sci-fi with any colour)
Stage six = When the ink has dried, give them some gel pens (there are some lovely colours, including various metallic
ones) Sets of the pens can be picked up cheaply (e.g. Poundland) Use these to put a bit of detail.
Stage seven = (optional) spray varnish, to protect the figures from lots of handling.
It seems a lot of work, but in reality it isn't. For a group night I would do any conversion & undercoating the session before
the 'painting' , or do that bit beforehand, for them.
When they get a bit more confidence they can try more detail, but the above will get THEIR figures onto the table.