Back when..... I last rode courier (messenger) there was no such thing as a mountain bike and my last courier bike was a Shogun roadie loaded with (really top of the wozza! ) shimano 600 gear and Araya rims, cost me $550, big biccies in the early eighties. However, were I to do it again, unlikely if not impossible, I would buy something like a Giant Yukon or Iguana and fit it with slicks as suggested. You want something tough with all the brakes you can buy disks, disks, disks. You wont use the granny gear but you will use all the rest of them. Front suspension is a must, your forearms and wrists will thank you for it at the end of a long week. If your tax laws are like ours and you are doing this legitimately then you should be able to write off the entire cost of your work vehicle and its maintainence as part of your costs of doing business, also your wet weather gear and general cycling crap. You could really push it and try to claim a couple of subscriptions to quality "industry" mags or even entry to bike shows!. Well maybe. The point is get a mid range MTB and write it and everything else you can off on your tax bill, at the end of the year sell it and buy a new one, you'll need to, it'll be rooted . Cheers!