The 2 biggest or relavant issues wrt usefulness of radars are IMHO are:
1/ Power drain - the older radars used to be like 20-30 amps continuous drain. Quite a bit of power drain for many boats. The new ones are like 4-7 amps. Quite a reduction. Depends upon what amperage your outboard can generate or whether or not you have an inboard/outboard with an alternator. So check that out 1st - and see if your outboard magneto can provide the electrical power needed. The outboard must be operating at a particular speed before any excess current is available, and in reduced visibility - so is your speed and engine RPM. I would think larger outboards would have larger electrical outputs in amps. need to check what these are though.
2/ Definition/sensitivity - depends upon both height and width of the antenna (as Sr. SQ posted), and the radar band used (e.g.: S-band 10 cm wavelength 2-4 GHz; or X-band 3cm wavelength 8-12 GHz). The longer the wavelength (the lower the frequency) - the longer the radar echo distance you get, but you loose on definition. Seems like today - with the small boat fleet radars - it is almost exclusively X bands with the exception of the Broadband Radars. Today - they package radars as 12, 24, 36 and 48 mile radars - combining power times frequency in one rating.
The newer radars commonly loose some sensitivity due to their small, compact antenna size and reduction in radar output. It may or may not be an issue for you. You loose sensitivity for compactness. It's a trade-off - and depends upon your needs (e.g. how fast you travel, how often there are logs, deadheads, and other non-metal boats operating in the area - and how often you go out, and how frequent there is fog rain and sea clutter- and how much weight you want on your wheelhouse and how much space you have). Basically what your needs are. The older radar antennas are often quite large and often do not fit well on smaller wheelhouses.
my $0.02 worth - for what it is worth. Don't forget - a radar reflector is a cheap investment for everyone.
