A fixed base router is preferred. You don't need high horsepower, but I would choose one with a 1/2" collet and one that is as light as possible. If you do much dovetail work you will also want to have 2 routers, one for the dovetail bit and one for the straight bit. You can make them with one, but setup time will increase. You should also seriously consider the Leigh recommended 8mm bit shanks over 1/4" because the 1/4" shanks have too much flex under load to make high quality dovetails. 8mm bit adapters are available from Leigh or other sources to let you use the 8mm bits in 1/2" router collets.
I use two DeWalt DW618 routers on my Leigh D4R and have decided that I prefer the D handle base over the fixed base for dovetail work because of the trigger grip, but the standard fixed base will work OK. A plunge base is not recommended, as you need more precise depth control and many of the router bits used have larger cutter diameters that are larger than the collets that are used, so they cannot be retracted through the collet.