At this point, you have probably understood that DwarvenWorld is not keyed to any rule-set; we personally use a custom set of rules we think fit the setting; however, if you want to employ something more traditional, we advise you to follow some guidelines:
Reduce and restrain the magic-using classes; for example, a troubadour (the character whom you'd call a bard in an ordinary RPG) in DwarvenWorld should not use magic, and one of the rare magicians certainly wouldn't leave his profession temporarily to train as a fighter
Set roleplay, and social interaction, rather than combat as the main focus of the game. To build an equivalent of an existing system, mastering in DwarvenWorld is very similar to mastering a mediaeval version of Call of Ctuluh: conspiracies are ripe in this decadent world, while monsters and magic rare and dangerous.
Rulebooks with a careful treatment of skills probably fit more the spirit of the setting, but are not strictly the only ones you can use.
That said, there are several rule-sets you can consider adopting; among these are:
Dungeons and Dragons (D&D, now out of print; it is an extremely simple system, but it suits first-timers)
Advanced Dungeons and Dragons (AD&D) 1st, 2nd and 3rd edition (the first two are out of print, while you can download the main rules of the latter by following the link given)
RoleMaster (by IronCrown; they do not, however, provide it for free)
The Phoenix Command Combat System (PCCS, by Leading Edge. It doesn't consider magic and it's extremely complicated to use, requiring a lot of work to adapt it to the needs of a fantasy setting. It is, however, extremely detailed and realistic as far as combat goes, and therefore ideal for a certain category of fanatics)
Generic Universal RolePlaying System (GURPS, by Steve Jackson games. This is probably the ideal system, but you'd have to sort through the countless books they publish to find a fitting one, and it's quite a costly activity)
The Basic Roleplaying System, which is, more or less all downloadable here (it is, as the name entails, sort of a down-to-the-ground ruleset)
The Hero System by Hero Games once apparently a very good set of rules once, it is now out of print and waiting for a fourth edition. It will not be, in the least, free
