I would suggest you get a limited set of colours, and the SAA do artist quality paint in tubes that will last a long time, and are very well priced.
With my beginners groups I use a basic set as follows, raw sienna, burnt sienna, burnt umber, ultramarine blue, cobalt blue, lemon yellow, auriolin, alizarin crimson and/or rose madder, and finally a colour the SAA doesn't have, perylene green. Play with the colours in mixes of two colours only to see the huge variation obtainable. Greens are sometimes awkward, I don't use pery' green straight from the tube but good variety can be obtained with the yellows, and a really good dark green with burnt sienna.
Best of luck