HANDLES: I personally use the No. 3 Handle, as it’s a good all-rounder. It fits a good variety of blades for most modelling jobs. Occasionally a No. 4 (larger handle is useful, but for starters – go for the No. 3).
I did not see if Swann & Morton were sold on Amazon.com – though I would say that they are my brand of choice, as they seem to have better manufacturing quality – the handles do not corrode and the blade-groove is well defined. Similarly the blades seem less prone to shattering, but with care either are fine.
BLADES: Clearly there are a huge variety of blades available for Scalpels, as they were devised for a variety of surgical procedures. However, here are a few tips on the 3 I use the most. For this I will cite a particular (UK) brand – so please observe the blade shape if you wish to purchase elsewhere.
No. 10A Blade – a squat straight edge for general use.
No. 11 Blade – a long, thin straight edge for fine detail/long cuts in hard to reach places. More fragile than No. 10A and hence will bend/snap with excessive pressure.
No. 10 Blade – a curved blade, for cutting soft materials such as rubber or foam. You will note that a straight edge will tear a soft material, whereas a curved blade will not give a straight cut in a hard material. This is why surgeons will use a curved blade (not a straight blade) to make the majority of incisions into flesh. Sorry if that was more information than you needed :p