Steve is spot on, excellent results can be obtained from just paint and pastels, but the branded products do take a lot of the work out of it for you and can provide some subtle differences.
I would suggest that if you are using acrylic paint/varnish then you are better off using oil based washes and weathering effects or visa versa.
Some effects such as panel line washes are best applied over a gloss coat, others such as pigment over a matt coat.
Last night I was working on the weathering for my F4U Birdcage (hopefully I will post it later this week when complete)
I used AKI weathering products:
Panel wash for grey/green aircraft (the F4U is blue/grey-blue
Dark Brown wash (inside and some outside streaking and effects)
Dust effects wash
European Earth pigment (to enhance the dust effects working with the wash as well)
Dark Earth pigment
Kerosene Leaks
Oil Leaks
Smoke pigment
I have also used the Ushi Chrome powder and some MIG Oil Brushers, Blue and Basic Flesh (latter for some paint wear to show the pink primer while the former was to show some tonal variation in worn areas)
Using these also helps getting some depth and texture in the appearance. Some, like the panel line and basic washes plus some pigments were used before the final matt coat. After the matt coat I used some pigments (for an enhanced dusty appearance) and the metal powder to get a metallic sheen in worn places, plus the kerosene and oil washes to make sure there was the right 'sheen' on spills and leaks. The latter are particularly good as these two washes really do look like oil spills or kerosene leaks which is not just about colour.
The Corsair I am building was quite a tired and well worn aircraft so it gives me plenty of weathering scope.
The best thing to do is try out different things and experiment to see what suits you best. You do nto have to spend a fortune and you can get good results with just oil paint and pastil chalks.