Choice of lens depends:
i. what focal length
ii. what type of photography
iii. what is crop factor on your camera (eg. M8 is 1.3 crop factor, M9 is FF)
if "walkabout lens" means street photography, then you are mainly choosing between 50mm or 35mm focal length. 50mm is more inclined towards portrait, whereas 35mm is more inclined towards landscape. Both are widely used for street photography. The difference is about 2 steps forward/back. If you can overcome this 2 steps difference with your feet, either will do fine.
WHAT FOCAL LENGTH TO BUY
- To achieve 50mm focal length, you need 35mm lens (M8) or 50mm lens (M9, M4, M6, MP), depending on your camera.
- To achieve 35mm focal length, you need 28mm lens (M8) or 35mm lens (M9, M4, M6, MP), depending on your camera.
WHAT BRAND TO BUY
After you know what focal length lens to buy, you then need to decide what brand to buy. Generally, price range from most expensive to least expensive in the order of Leica, Zeiss, Voigtlander. The difference in quality is debatable - I find most of the time the main consideration is the "character" more than the "quality". All 3 brands give better quality than most commercial grade lenses out there for DSLRS. To judge what "character" of lens is good for you, look at
http://www.flickriver.com and look at images based on "lens explorer".
NEW OR USED
After you decide what brand to buy, you should decide whether to buy brand new or buy used (2nd hand).
You should find that brand new lenses are usually either out of stock or priced at a premium.
Used lenses are usually a good bargain because (i) most people keep their lenses well and (ii) used lenses cost less; and (iii) retain value better. In worst case scenario, if used lens is dirty, just send for CLA costs about approx 250 bucks.
good luck