Downpipe and PLOG are SUPER easy tech wise. The biggest issues are getting the car off of the ground enough to get some leverage on the downpipe bolts (which WILL be rusty). I had never done any sort of modding either until a month or so ago, so I didn't feel real confidant either. I never did get the downpipe bolts off, so I had to take it to shop for that, but it is 4 bolts, two rubber supports (look like FAT rubber bands) and the oxygen sensor removal and install (which screws out and back in easy enough). Like I said, VERY EASY tech wise, just a pain if you can't get under the car enough.
For the PLOG, it was absolutely the fastest and easiest mod I've done so far (except maybe the T-Stat swap or the dog bone flip). I was surprised at how easy it was. Get a deep socket 13mm and a 13mm wrench and have at it. It was something like 10 bolts or nuts (it was a combination of the two) and that was it. Just disconnect the spark plug wires (noting their locations for reinstallation) and then the bolts and nuts. Be careful around the dipstick though. Then just lift out the old and the gasket, put the new gasket on and put the new in. It took me something like 20 minutes to swap. Oh, and put the plug wires back on. Last tip. I found it easier if I removed the front passenger side engine support (dog bone). so that I could get a ratchet into that area easier.
Also, for the rusted bolts/nuts, get some PB Blaster. It is a spray lubricant and rust "destroyer". Spray the bolts/nuts each day for a few days before doing the work, and then again about 20 minutes before removing them. Makes life a LOT easier. You should be able to get it at about any automotive supply shop.
As for a good shop in Mass.... I wouldn't have the faintest idea. Sorry.