Perhaps you can find a long-barrel S&W Model 29 in .44 Mag that's used and then add a rail and have it ported.  This would cut your acquisition cost and allow the other features to be done by a good gunsmith. 

 

There are several outfits that do porting of pistol and shotgun barrels (Magna-Port is one).  Porting is done with EDM (electro-discharge machining) that does not leave any burrs as are caused by traditional metal working techniques.  For pistols, there are usually two ports (or a series of holes) EDM'ed on either side of the front sight near the muzzle.  Porting uses the propellant gases to help keep muzzle rise minimal to speed a follow-up shot.  As you know, the un-ported .44 Mag has considerable muzzle rise.

 

The long barrel handguns in .44 Mag a heavy and require two handed support for accurate shot placement.  Use an improvised rest where ever possible.  A 7-1/2 to 8-3/8 inch barrel is probably the maximum length I'd go with.  See if Pachmayr makes grips for the pistol.

 

Bottom line: look around for a long barreled .44 Magnum that's used before you buy new.  You may get a bargain, and it won't break your piggy bank to add either the porting or the rail.

 

Trivia:  The film "Dirty Harry" used the S&W Model 29 in .44 Magnum with the 8-3/8 inch barrel in the scene where he draws the gun for the first time.  However, filming was done with possibly two other guns: either the S&W Model 57 (.41 Mag) or Model 25 (.45 Long Colt) -- either of which looked like the Model 29.  According to one source, filming the S&W Model 57 in .41 Magnum was used to substitute for the real Model 29 -- this being out-of-stock at S&W at the time.  Either the .44 Mag or .41 Mag would have required the film company to "roll their own" blanks.  So, it might have made sense to use the other stand-in: the Model 25.

 

The other stand-in for the Model 29 was the Model 25 in .45 Long Colt that could use the 5-in-1 Hollywood blanks that were readily available for purchase.  The prop Model 25 (or Model 57) had a 6 inch barrel that was not noticable in the film.  Both Models 57 and 25 were indistinguishable from the "real" Model 29 during filming.