Sorry, long post, but want you to have all the info.

Ok, I've been searching and reading tons of posts, but I'm not sure where to begin. I have a "new" 2000 Auto 4WD LT Blazer with 155K on it. Doesn't look like anything has been done to the motor before I got it. When the outside temp is above freezing, it starts just fine. I replaced the fuel filter already, and removed the throttle body and did a thorough cleaning of it and the IAC valve. I have also changed the coolant and thermostat and flushed all the lines/heater core/etc. (It wasn't producing heat from the core, or getting the engine up to temp, but works great now). There are no codes present, other than an air bag light.

Issue: Hard or no start condition depending on temperature.

-About 15-32 deg F, engine will start, then die right away unless I give it gas for a few seconds after starting.
-About 5-15 deg F, engine will not start, but if I use a hairdryer and heat up the coil and ignition module area, it will start, but I still have to give it gas right after for a few seconds.
-Any temp lower than 5 deg, I can not get it to start even with heating up the ignition components, even if I shoot starting fluid in the throttle body.

Here's where I started.

-Test took place in 65 deg heated garage...
-Rented a Fuel pressure gauge at AZ.
-Attached to fuel test valve.
-Jumpered the test port to run the fuel pump while vehicle off. Got 61 PSI. --Turned off pump and watched gauge for 10 minutes. PSI dropped to 55, then maintained pressure for 10 minutes.
-Started just fine in heated garage.

I'm not opposed to buy parts for this vehicle, but wonder where to start.

-Should I just start with plugs, wires, rotor, dist cap, coil? I would love to start with just the coil, but would have to order an AC Delco one ($20 on AZ, but would take 10-15 days). I can get a cheap brand for about $30 locally. Is it worth trying?
-Or do I plan to tear into the intake and do the upper/lower gaskets, new spider/regulator, and clean everything while I'm in there?

***Other symptom: When vehicle is all warmed up, when I rev the engine and let off quickly, it sounds like backfiring a little, and the RPM's drop to 400 like it's going to die, but recovers and idles at about 550-600. ***

I do have a Panlong OBD II bluetooth code reader tool with Torque Lite on my phone, but don't know what half of the gauges really mean.

Any input is appreciated, thanks in advance!