Depends how many other Dreamcast games you'd want to play. The reason I say this is there are two basic ways for you to go with arcade controls and Dreamcast. One is it hack DC controllers or buy some pre hacked controllers (recommended) or buy an Xarcade BYO kit and Dreamcast adapter. The Xarcade method gives you both analog and digital controls where as the hacked DC pads will typically only get ya the D-Pad and the buttons / triggers.
If just Marvel vs Capcom 2 and not planing on putting any other console in the cab then contact Haruman on the board and buy a pair of hacked controllers for $30 this will get you 2 DC pads with wired leads ready to connect to arcade controls and still allow you to use the Dreamcast VMU's. If you go Xarcade which will run ya about $80 you can buy additional adapters for Xbox, PS2, Gamecube for $20 each and simply add the console and the adapter and bam you have a common contol panel for multiple consoles.
The other piece is video. If you are using a TV in your cab then that should be clear. If using a computer monitor then $15 will get you a VGA adapter (much better picture). If you wanted to go multiple consoles you can get a KVM that support audio and some RCA adapters and you can switch between systems very easily.
You can do this the complicated way, the easy way, and the adaptable way....
I have both a NAOMI system and a cab with the Dreamcast. While the NAOMI is authentic and gives you some bragging rights, the Dreamcast I have in the cab is incredible. 90% of the NAOMI games have been ported to Dreamcast so you can play the same games at 1/10th or less of the cost.
NAOMI Marvel vs Capcom 2 the cart alone would run $225 - $300 depending on current market and then you'd still need the rest of the NAOMI system lets just say thats another $300 - $800 depending on how far you want to go with NAOMI.
Hop this helps,
-Goz