Not sure what you mean by "DOS", but in Amiga-emulation you
got basiclly two possible routes:
A) UAE, runs on MS-DOS,Windows and Linux, but the Win-version
is the most advanced.
The emu itself is free, but you will need an ROM-image which you
can either extract from a real Amiga or buy with the Amiga-Forever
package. UAE is a full emulator, including the chipset. making it
possible to run anything from old games to brand new apps.
B) Amithlon runs on it's own (on top of a specially modded
Linux-kernel), costs ~150 $/Euro, includes everthing you will
need, doesn't emulate the chipset (so no old games), but is twice
(sometimes even more) as fast as UAE. But beware and check if your HW
is compatible with it.
There is also some legal fighting over Amithlon going on, but since
none of the parties seems to have the guts (or whatever) to take it
into court, I wouldn't see a prob in buying it.