trons, I also have a 98 and have fixed the same codes over the past month with the exception of 304.
IMRC vacuum accuators - both 1538 and 1537 will show up with one bad diaphram. Pull and plug the hose on one, start and rev the engine and check for proper operation on the other. Repeat process for other. $10 says when you have the front one plugged the back one will operate, but not vice versa. That means the front one is bad. The part is available at fivestarford.com for the prices you indicated. Call and ask for the internet parts guy. Also buy the $5 clip/bushing. I found it real easy to change the front one with the alternator off.
171-174 - on a pre 99 these usually just mean a vacuum leak, or dirty MAF. What the computer is telling you is it can reconcile the the reading in the o2 sensors with the amount of air coming through the MAF. This is probably because air is getting in somewhere after the MAF. I would not be suprised if it is related to the vacuum accuator above. The 98 does not have the 2 part intake and the manifold leaks assoiated with the newer models.
I would fix the imrc vaccum accuator first, and see if you get rid of the 171 and 174 codes.
That being said if you still have 304, you are going to pull the intake manifold any way to clean the egr ports, at this time you might as well put in the 6 gaskets.
As an aside, I had to chase down 2 other vaccum leaks on mine. The elbow off of the top of the intake developed a leak, and the new pcv valve I put in kept popping out.