A wetsuit which I could borrow from a friend, not specific to swimming, or a speedo fast skin covering from ankles to shoulders, no arms?

a non-specific wetsuit is not a good idea for suface swimming. Then again, is the fastskin really a wetsuit or is it one of those high-tech swimsuits? If the speedo is really a wetsuit, I'd go with it, or if you are swimming in warm water go with it, but otherwise, Id suggest trying to rent a swim specific wetsuit.

What's the event? How long is the swim? Ocean? Lake? What temp are you expecting?

No question - The Speedo Fastskin
Don't even think of using a non-swimming type of wetsuit for a triathlon.

If you want to use a wet suit - call around to different running/triathlon shops in your area and ask about renting a wet suit. Fleetfeet near me will rent Ironman Stealth and Instinct wet suits for like $18.00. They offer both Longjohn and Full styles - Choosing to rent allows you to try different sizes and types of wet suits prior to spending the big $$ to buy your own. However, make sure you reserve it way ahead of time prior to your Triathlon or you will find them all reserved by other triathletes.