randog, the 517 is the same tuna as the 717, except for one useless feature, which i forget what it is... ask ed, or ask at the yahoo tuna mod site.
as far as ed's prices, $200-$300 for mods & a complete alignment seems quite fair to me... also, prices are similar for other folk, like stephen sank and bill ammons. ed hanlon's prices on awreddy-done-up stuff generally reflect the going ebay rate for the tuna itself.
the mods generally consist of filter replacement, upgrade of diodes & capacitors, etc. also, disconnecting the 300 ohm antenna hook-up & using only the 75 ohm, makes for better sensitivity... again, ask ed, or the yahoo tuna site. or, read about the mods done to the kenwood kt-7500 on fmtunerinfo.com
the more ya read about folks' experiences w/tuna, the less you'll want a mr78, so relax about that. a modded mr71 may be a different story, tho...
ya definitely need a good antenna on a rotor; spring for an aps-13, or get a winegard hd6065 if the funds are tight. i have had both, & performance on the winegard is close enuff to the aps-13 to make it a steal, imo...
hth,
doug s.