Really this is a non-issue. Electrically converting video signals between Displayport, DVI (Single link), and HDMI is not a big deal. For any of those combinations, you can skip a separate adapter and just buy a cable with the correct ends for about $10 from Amazon, Monoprice, etc.
If you are using a HDMI monitor that has built in speakers and want audio out over the mini Displayport, you will need to get an adapter that supports (mini) DP->HDMI + Audio, which may be a pain to find and probably about $40. HOWEVER, the unibody mac minis have an HDMI port built in, so you can just use a $5 HDMI cable.
There are only 2 gotchas I am aware of.
One is using a 30" monitor ( 2560 x 1600, or anything over 1920x1200 really ) that has DVI input. Regular DVI tops out bandwidth-wise at 1920x1200, so they made DVI-D (Dual Link), which uses an additional 9 lines. To go from (Mini) Displayport to DVI-D, you need an active adapter, which costs $70-$100, and usually needs a USB port to plug into as well.
The Other is plugging into a 4K (or 5K) monitor. I've not tried this yet, so my knowledge is lacking, but we're still in the early stages of 4K+ support and pre-2014 minis may be limited to 30fps or 24fps (or not at work at all) depending on the combination of connection, monitor/TV set and Mini used. Pretty soon, everything will be caught up and have displayport 1.4a/HDMI 2.0/etc and just work for 4K @ 60fps. etc.