I actually acquired a LaserWriter Plus nine years ago. It needed new toner, and even back then it was discontinued by the manufacturer and going for well over $100 from third party companies (and I believe that price was for a remanufactured cartridge, not a new one). I paid $15 for it; it had come from a local nursing home where my friend's mother just so happened to volunteer. I still remember lugging that beast inside my basement with him! I never got around to getting a toner for it and it sat in my basement for a long time.
Four years later, I traded it and a Mac IIx to someone for a boxed Mac 128K. He wanted an old printer for parts and his wife wanted the 128K gone for whatever price it took. At least there was a happy ending to the story of the LaserWriter that sat unused for years.
One of the huge drawbacks of the original LaserWriter and LaserWriter Plus is the Canon CX engine. If you print a large solid black area, it will print as a pattern and not as solid black. This is a limitation of the CX. The SX engine used in the LaserWriter IIs fix this problem. Keep this in mind if you are absolutely intent on getting a LaserWriter Plus and doing some serious printing with it.
Your best bet is to get a 4/600 PS. Toner is cheap for this model. So are LocalTalk/PhoneNet boxes; you'll need one to connect it to a Mac. I've used it with the LaserWriter drivers from 6.0.8 and also with newer versions of the system on Macs from the Plus through 030s. It's fairly lightweight and ships nicely. Look for one with low mileage (if you can find one under 10K pages you're really in business; otherwise search for under 50K; the lower the better). I paid $15 for mine and I believe the last toner cartridge I bought for it was $35 on eBay. It can't hold a hard drive, is fairly slow compared to other lasers I've used, and only has a Mini DIN-8 LocalTalk port, but is built well and prints up there with the rest of them. One slight problem with it--you'll need to find a very flexible power cord to close the power cord door on the printer correctly. Most of Apple's OEM cords don't work right with this unless you're lucky enough to have one that came with the printer. Standard black or gray cords usually aren't good here either; you need to find one that's thin and very flexible (or else you can simply remove the door, which I did until finding an ancient beige cord that worked perfectly).
The 16/600 PS is nice too, but it's larger and not as available online (due to its huge size and astronomical shipping costs). This one has a parallel interface, a hookup for an ethernet jack (you'll need the adapter), optional paper trays, a faster print speed, and no power cord issues. If you can find one and have the room for it, spring for it by all means. I can also recommend the IIf and IIg (the second generation LaserWriter IIs and some of the most durable printers I've ever encountered in the field) and the LaserWriter Pro (forerunner to the 16/600 PS), but again, you're facing huge shipping costs.
