Having a ham ticket is one of the essentials of this balloonacy stuff, since that's the only viable medium for communicating with the payload.
The next step is to get familiar with the FAA Regulations related to this activity - as stipulated in 14 CFR Part 101. You can download that regulation from www.faa.gov, or better yet, an annotated version from the EOSS web site. While you're there, download the EOSS Handbook - it's a bit dated but is still a good starting point.
EOSS rigidly adheres to a "no trespassing" policy during its recovery operations. Once we've identified the landing site, we seek out the landowner and get permission to go in for recovery. SO far, our experiences over 71 flights and recoveries have been quite pleasant. The vast majority of owners are tickled to help us out once we explain who we are and what we're up to.
Re your 12 lb payload: be careful. If you can keep your individual payload packages under 6 lb, then you'll be exempt from FAR 101 (see para 101.1(a)(4)). If you've got AM ATV aboard, getting 10 to 20 ft of separation from your more sensitive electronics, like GPS receiver, may save a lot of EMC management pain as well.
Unfortunately, there is no Payload 'R Us; although we use off-shelf radios, cameras, TNCs, GPS etc, the systems are custom designed and built and programmed specifically for light weight and survivability. See the foamcore paper on the web site for tips on housings. EOSS currently has seven separate balloon payloads that we select from for a given mission - some pix are on the web site as well.
And EOSS will be more than happy to give you more help, but we'll have to leave the sweat equity up to you. This is definitely an homebrew area of ham radio!
The balloon beacons will have to be IDed with someone's callsign, and they should operate on interference free frequencies. EOSS and other balloon groups have found 144.34 and 145.600 to work well. 147.555 also works nice, but when we flew out of Kansas last year, we learned that was used by the Skywarn folks - never can tell. The 70 cm FM simplex frequencies below 446.000 also have worked pretty well.
Re power: our ATV transmitter actually runs about 1W PEP AM to a little wheel (5 dBi) - and using an 18-el yagi with an LNA (preamp), we get P3+ out to 50 miles downrange. Line of sight is a magic thing! Our crossband repeater runs just 180 mW on 147.555, and it supported a QSO of 530 miles between Garden City KS and Riverton WY.
After the payload lands, though, signals go WAY down! So you should have a T&R team deployed near the predicted landing site (use Balloon Track for Windows) and use APRS beaconing at least every 60 sec during the flight. If the T&R team gets LOS on the APRS, it becomes a DF foxhunt, so a decent ERP from a self-erecting 2m antenna is a real help. BTW, don't rely on 10m for a recovery beacon - ground wave props are way worse than on 2m.
For packet, the KPC-3+ is a proven performer - stripped of it's housing, it weighs about 100 gm. And the Alinco DJ-C1, at 70 gm w/o its Li battery, has also proven solid. In fact, nearly every HT we've tried survives the chill. If in doubt, cold soak at zero Celcius in a food freezer for a few hours - this pretty well emulates the min temp inside a 0.28" foamcore housing with active electronics running. Or you can test the whole payload in an ice chest with dry ice, but we've learned that's an unnecessary expense.
BTW, the KPC-3+'s analog telemetry is useful for down linking battery voltage, barometric pressure, temps etc. We brought out the two unused ADC inputs to the DB-25 to get us 5 analog channels. We use DTMF for commanding, but an embedded processor on the KPC-3 main port ought to be a whole lot more versatile.
Be careful to pick a GPS that will report altitude to over 30km - many if them are programmed to shut down above 65,000 ft., and some even lower. The Garmin Etrex and Motorola engines do work. We use the Rockwell Jupiter engines on ours, but they're out of production.
Another EMC trick that we've just learned is to avoid using the payload circuit common as a counterpoise for a 1/4 wave whip. This has addled the micromind on the Dionics TinyTrak APRS beacon gen. Use a flexible J-pole, like the MFJ pocket rollup or a ground plane to make life easier.