What I would suggest is either build or buy a seperate small shed that is air-tight, set up an air-conditioned cooling system, this way you keep not only the dust out but you keep the server cool especially in the summer time.
The shed wouldn't cost but a couple hundered and the a/c another couple hundered. Overall, it may seem expensive to begin with but in the long run it would seriously pay for itself, both in time and money saved from having to replace filters and/or computers.
The only other suggestion would be to set up the server remotely and access it through either a DSL or Cable connection. This would be cheaper unless you have no place to set it up at.