The Remington "Special Purpose" is just a Wingmaster with a parkerized finish. Most barrels will fit.
Other than picking up a used barrel and cutting it off, Mossberg sells Remington-type 18" Parkerized barrels at a very good price. A little comparative shopping will get you one at a very good price.
Mag extensions savings could be had by using one of the new plastic ones. However, this could be a bad idea, as I've no experience with these, and I don't know if they will stand up to use. I'd recommend getting a Choate or Tac-star WITH a barrel clamp. Choate make a parkerized unit that will match your gun.
Shortening the stock is no huge job. Getting it done shouldn't cost much, and you can do it yourself. If nothing else, most woodworking shops can do it, cheap.
Ghost sights, (or any rifle-type sight) makes hitting that much easier, no matter the range. I usually use a "flash" sight picture, as I look over/through the sights. Since most Remingtons don't fit me, I tend to shoot quite high without sights.
Look at
www.brownells.com Brownell's carry just about all decent shotgun accessories including sights.
Shotgun sights CAN be attached quite well with soft solder, as a do-it-yourself project. While not as sturdy as a factory brased-on job, I've had customers using soldered on sights for many years without problems.
Bottom line: If you have any hobby skills you can have the gun you want without a huge outlay. Since you can do the job one item at a time, you don't have to cough up the cash all at once.
Go for it.