Malebolgia's recommendations are very good (I like W&N Series 7 too, and use a size 1 for just about everything - even the highlights on eyes).
However, a typical size 1 W&N7 brush will run you about
£7.00 plus shipping as an average good price.
If you are learning to paint, then I'd suggest some cheaper but still decent sable brushes for miniatures. The Vallejo red sable brushes are very reasonable, and you can get them from SnM Stuff
here. They sell the same brushes as Vallejo, but in their non-branded form straight from the manufacturer, which makes them cheaper.
For painting terrain, I'd suggest using craft acrylic paint rather than the model acrylic paint as they are much cheaper and you won't need the same properties as for painting figures. Any half-decent art store will carry several economical ranges, so have a look around and get a sensible selection.
Brushes-wise, get some cheap but half-decent watercolour brushes for painting terrain. They will have a hard life, and whilst you do want something that will be a good and useful tool, you don't want to burn through expensive brushes where they are overkill. I get all my brushes (including the W&N7s) from Ken Bromley, who do a decent budget range of watercolour brushes
here.
If you want to drybrush any of your terrain, get yourself some brushes designed for oil painting, as they are stiffer and will not only work better but last longer too.
These perhaps? I like natural brush bristles myself, but others prefer synthetic bristles. Try them out and see what you prefer - you can always keep the one you like least for applying glue with!
Finally, assuming that you are new to all this painting models malarky, I'll leave you with a few tips that I found enlightening when I started out and made a huge difference:
1) Always use a bigger brush than you think you'll need, and always use brushes with sharp points for detail work rather than teeny-tiny brushes. The bigger brushes hold more paint and don't dry out, which makes it easier to do neat detail work. The sharpness of the brush tip is what gives you fine control over detail work, not the brush size.
2) Thin your paints. Rather a lot in fact. It is better to apply the paint in three thin layers than two thicker ones as you will get a smoother finish and will preserve the detail of the model. Load the brush with less paint to improve control when applying it to the model - easier with thin paint. The thin paint also dries less in the brush, which when combined with frequent brush rinsing, extends the life of your brushes.
3) Don't just go with the the 'easiest' technique when you start out. Try shading and highlighting as well as drybrushing and inking, and see what the differences are. Take your time, and have go at different methods to see what you think you prefer. Some techniques have obvious advantages (like speed) over others, but it is always harder to be open to new things once you're stuck in your ways!
