You have several choices when adding a pickup to an acoustic. You can add a magnetic pickup that fits in the sound hole without modifying your guitar. These pickups work by picking up the string vibrations, like a regular electric guitar, so your acoustic sound may not be projected. Magnetic pickups are usually inexpensive and you may or may not like the sound.
You can also add a piezoelectric pickup that works by amplifying the vibrations of the top of the guitar. Piezo pickups send a very small signal, so you will usually need a preamp to bring the signal up to the levels that your amplifier can use. Some piezos will mount directly to the top with double-sided tape, but most (from Fishman, Martin, etc.) require installation by a guitar tech. Unfortunately, the cost of the piezo pickup, preamp and installation can be expensive. Unless you have an expensive acoustic, it may be cheaper to buy an electro-acoustic that already has the pickups in place. Piezo pickups are very sensitive and can suffer from a "quacky" sound. I believe that this sound is caused when the movement of the top is out-of-phase with the movement of the speakers. Some of the acoustic pickups come with a phase-reversal switch to combat this occurence.
There are a lot of good electro-acoustic guitars available. When I decided to amplify my acoustic, I ran into the same problems that you are having. When I investigated, I found that I would spend several hundred dollars to add a pickup to my old flat-top. I ended up shopping around and buying a Yamaha LD-10E that has both a piezo pickup and an internal microphone. I can blend the two sounds and adjust the equalization to help combat feedback.
Acoustics are always more sensitive to feedback than solid-body guitars. You will never be able to compete with a Les Paul through a cranked Marshall. The secret to controlling feedback is controlling stage volume. Play loud enough to get the sound you want, then mike the amps and instruments and use the PA to bring the audience sound level up.