**FYI ** As SRK policy states, no one is allowed on the forum to give out or give links to Roms, ISOs or the like, regardless if you own the physical game or not. What you claim as legal is actually a huge disputed legal gray area, which varries from state to state, country to country. I understand in some countries its a serious criminal offense. There are a number of forums that can help you, unfortunately for you SRK although neutral on the topic of emulation it self, its completely apposed to any sharing of such files.
For the Arcade version you need a arcade Emulator like M.A.M.E. (you have to Find a version that would support the game), the ROM image in question and a NAOMI BIOS file. Note that a lot of Arcade Rom dumps are corrupted or improperly formatted. Too Often Updated versions of MAME will sacrifice game compatibility for stability.
For the Dreamcast Version, yes NullDC should work. Make sure the Rom you receive is not corrupted or improperly formatted.
As for the DC save files? You are own your own, it is going into territory most people on SRK will not go into on (at least on this forum). Best advice I can give there are disc images that hold various game file info, that write the correct save file to your virtual VMU for the emulator or you can search for Action Replay/ Game Shark codes to enable the unlocks you want.
That is as far I can and willing to help you on this forum. The Rest is on you. In this matter most of SRK, or any forum would recommend you google the info you want.