VHF and GPS -- Opinions...
The fittings on fixed-mount VHF units pretty much universally connect, including to the back of the radio.
If the existing fittings are not corroded, you can just use them as is. If mildly corroded, try to clean them up (small, relatively soft, wire brush). Same thing at the antenna end.
Install the radio. If it works well, you are good to go. If not, try using your radio on someone else's boat (that has a known working radio) ... process of elimination. I've replaced several radios on multiple boats, and so far no problems. (Though I do know a few people that have had problems.)
VHF radios are pretty much line-of-sight, so distance for communication is more a function of the earth's curvature and "stuff" in the way (buildings, mountains, sand dunes, etc), than anything else. Sailboats have a distinct advantage in height of antenna if they use a mast mount antenna.
For example, if your antenna is at 6 feet and the other guys is at 6 feet, you'll get about 5-6 miles of coverage. You can look up the tables for distances in just about any write-up of marine VHF's. Truth is, it doesn't normally matter because 99% of us just use whatever we have.
Note all new fixed mount VHF's have a capability called DSC, which most of us will never use except for its Emergency function. You can even hook them up to a GPS (not an adventure for the faint of heart) and that emergency button (and other functions) will supply your Lat/Lon to with in about 500 feet! Most handhelds do not yet have DSC, but it is becoming more common.
Handhelds are convenient and as noted above, less expensive, but not a lot less. As prices go up (fixed or handheld), you get better selectivity (picking out one transmitter) and sensitivity (picking up weaker signals), but those improvements go up much slower than the price. You also get other features, especially on fixed mounts, like a workable PA system and automatic fog horn signals. However, any VHF with a standard price over $120 bucks will do a great job. So will quite a few under that price level, and of course, they DO go on sale!
GPS... I agree with the above post that handhelds are fully adequate, have all the needed functionality, are often a bit easier to deal with, and are less expensive; though sometimes not much. On other big advantage to handhelds is that you can take them with you (the fixed mounts can be removed, but they NEED that 12 volt power supply to function. Besides that, most of us with "smaller" boats don't really have a good place to mount a fixed unit.
Almost all GPS's sold now come with (built-in) either US Coastal or US Inland Lakes charts (yup, the entire US coast, usually with the Bahamas thrown in). Even the new Garmin GPS 78 series has their better US coastal charts built in plus the capacity to add other charts on SD cards.
Most GPS manufacturers have some kind of software that will let you use those detailed maps on your GPS'es or on your SD Cards for trip planning. Very handly feature, as it is usually a bit of a PITA to plan routes directly onmost GPS units (excepting those big 12-16 inch screen jobs.
FWIW... I have both a fixed and a handheld VHF on board. The fixed stays on channel 16 and the handheld stays on whatever I feel like that day. MOst of my general communication is on the handheld from the convenience of the cockpit. If a signal is weak, then I might go below and use the fixed mount. If you are only going to have one, I'd go with the handheld first.
My GPS is a Garmin 76Cx handheld, which has a really good bright sunlight screen! If I had been able to wait a bit longer, I'd have gone with the new GPS 78 as it has built-in coastal maps, a slot for and SD card (more maps if wanted), AND a bif slug of built-in memory for things like waypoints, routes, tracks, etc. My 76 doesn't have internal memory,nor built-in charts, so my waypoints, routes, etc have to share whatever space is available (left over) on the chart SD Card.
BTW: My advice... never set your waypoints on top of or really close to, bouys, especially entrance (fairway) bouys. LOTS of people do that , crank on the autopilot, and go below for a beer or whatever. No sense piloting your boat to the SAME location dozens of other yahoos are using. Just set your waypoints a few hundred yards away.
Hope this helps... Happy Sailing!