I use Apple's separate SuperDrive on my Mac Mini Server without issue.
This is specifically designed for the Mini/MacBook Pro/Air - as extra power is fed to the SuperDrive. That is, you may very well discover that your third-party disc drive will not work with your chosen Mac.
My daughter's MacBook Pro with dual-core i7 processor runs along quite nicely with applications like Adobe CS5 Design Premium - so settling on dual-core for the Mac Mini shouldn't be an issue - especially since upgrading from i5 to i7 is an extra �80.
Even so, buying the Mac Mini Server, that has the added benefit of quad-core i7, plus twin 500Gb drives for �70 more than an upgraded standard Mac Mini with dual-core i7 is, in my opinion, a better proposition.
The caveat is that the better of the two standard Mac Mini's has a dedicated AMD Radeon HD 6630M graphics processor with 256MB of GDDR5 memory, while the lower specified Mini (and Server) have an Intel HD Graphics 3000 processor with 288MB of DDR3 SDRAM shared with main memory.
BTW, only buy the standard 4Gb memory configuration, as the full 8Gb upgrade is substantially cheaper from Crucial.