"how can i tell if my old antennas are any good and replace there cables?"
A working antenna uses the power that is sent to it by the transmitter. A bad antenna can't put the power into the air and so reflects most of it back to the transmitter. The measure of how well the antenna is working is the amount of power you send to it relative to the amount of power it can't use. This is measured as "standing wave ratio".
SWR is measured by a box that you insert between transmitter and antenna. You should have one. It is much better than a "radio check" and annoys the Coast Guard less. Shakespeare has a marine one for around $100, but any "VHF" meter should work.
A few radios have the swr function built in. Probably not yours, but you should check before buying or borrowing.
"i have two antennas can i use the other one for my fm radio?"
You are better off getting an fm antenna, they are pretty cheap. A vhf antenna works poorly with fm.
Shakespeare makes a box that lets you share an antenna between a vhf radio and an fm receiver. I have one and I don't like it, works like cr*p. Next trip to the boat I am installing a tv flying saucer and I plan to use it for fm as well. The fm band (88 to 108 MHertz) is smack between channel 6 and channel 7. Marine is up around 160 MHz.
"do i need an external gps antenna?it will be in the cockpit on a express."
Put the GPS antenna on the roof, clear of obstacles. It does not need to be high, like a communications antenna, but it needs a clear view of the whole sky. If you mount a unit with a built-in antenna down in the shade, it won't work at all.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida