98-01 - This is the first "production run" of the second gen LH cars, the 2.7 and 3.2 were dominate. That said, I've seen some early year LH vehicles with the 3.5, mostly 300M's and LHS's. Everything else was offered with 3.2's if they were high-trim models.
02-04 - This is the second "production run" of the second gen LH cars, the 2.7 and 3.5 were dominate. The 3.2 was discontinued. The 3.5 was reserved for "higher-end" models, like the ES, SXT, RT Intrepids and 300M's and LHS's. I've actually never seen a Concorde with a 3.5.
The Plymouth/Chrysler Prowler was only offered with a 3.5. Even though the engine and transmission are the same out of the LH vehicles (difference being they used a drive shaft system between the engine and tranny), the likely hood of finding a wrecked or parted out Prowler would be RARE, and I mean RARE.
Daytrepper's manual is definitely worth purchase if you have any question or doubt about the process. He went to extreme depth to make sure everything is covered. A friend of mine had a copy of it and I peaked through it, I'd consider it the LH engine swap bible.
When talking 2.7, it's an interesting topic. The motor is incredibly strong, reliable and well engineered, with the obvious exception. If they had designed the water pump so it was not able to leak directly into the main oil sump of the engine due to gasket or bearing failure, it would literally not been an issue-motor.
The issue with the 2.7 when it's water pump fails is a pretty signification chain reaction. If the engine was run, driven or left with the contents of the cooling system in the oil, you can be sure enough that any bearings which have come in contact with the mixture are damaged. The coolant washes them out. Additionally the timing system relies on oil-pressure, and the oil pressure would have been significantly reduced, running the risk of timing skipping chain leaks, or worse, causing the pistons and valves to crash. The common "engine sludge" complaints are the source of the water pump leaking and frankly are only the tip of the spear.
If you want to make your Intrepid into a bit of a performer, you certainly can - just don't expect to make it into a Lamborghini. In a nut shell, once you throw that 3.5 motor into your car with your original 2.7 transmission, the difference in final drive gears will give you a great kick-in-the-pants low end torque difference (keep in mind, you can change these gears at any time through a service panel). You can buy and make your own cold-air intake system, K&N offers ones. There are lots of exhaust systems offered as well. Aluminum underdrive pulleys also exist. Many small goodies that can make a difference.
If you really wanted to throw money at it, again this isn't a cheap venture, you could consider adding a single or twin turbo setup, or centrifuge supercharger. Again, requires lots of custom fabrication and can be costly, you'll also need to beef up the transmission too (Kevlar clutch pack).
There is also the stroker route. Many people are yanking their 3.5's, and throwing in the 4.0's crank and pistons and with the right know-how you have a 4.0 Stroker.
For now. Find a motor, and the accessories and go from there once you know how much you can throw at it.