1997 Honda Civic Help!!! Not sure what the Problems are??

Not counting regular maintenance I have been very fortunate in having to make only a few unexpected repairs since I’ve owned it. These included a new exhaust, new manifold (cracked) and catalytic converter (was not defective but attached to the manufacturer’s manifold so replacement was not optional), and finally CV joints. It seems that recently my luck has run out!! Initially the problem began with the illumination (constant) of the check engine light after starting the car one morning. As this immediately caught my attention, I paused to assess the other lights and gauges before proceeding to drive. Both the temperature and fuel gauges were consistent with normal operating levels, and no other indicator lights were lit. As soon as I began to accelerate I noticed the speedometer was going hay wire, darting between 0-120 mph, and sometimes riding upside down between 0-120) with no indication of my actual speed. This was more annoying then detrimental in my mind, and seemed pretty minor in the grand scheme of things and over the next couple of days no other red flags popped up, although I did notice the check engine light was not consistent in brightness, and seemed to almost flicker from dim to bright as I accelerated. Then, suddenly the car would not start after being turned off no more that 4-5 hours since last running. Initially I thought perhaps I had left my lights on and drained the battery (which was not the case), but after repeated attempts at jumping it with no luck in getting it to start. After testing the battery (relatively new, just a couple months past the 2 year warranty, go figure), it was putting out 11.5 volts but had a defective “deep cell” and needed to be replaced. After installing the new battery, the car started right up (the check engine light was still lit). Two or three more days went by without any problems, and then I noticed that neither the trip meter or the odometer were working (or if they were only at random, and more not working than working). At this point I bucked up for the “CANOBD2” diagnostic tester. After linking the tester to my car, it was unable to complete the testing of the “O” and “C” monitor status for some reason, but shot back four DTC codes. They were as follows: PO135, followed by PO500, P1298 and finally PO141. So now as barely a week has passed since replacing the battery and today I start it up, and let it run to warm up (30 degrees and snow all day today). About ten minutes pass, I get in to drive home and the “oil pressure” indicator light and the “SRS” indicator lights are both on! I immediately turn the engine off, pop the hood, and proceed to check the oil and fluid levels, everything seemed to be at appropriate levels. So I get back in it to start it back up and go home and it won’t start again!!!????? Any advice on what’s really going on in there? It sure seems like a lot of problems in a short time frame, and likely several pretty expensive repairs. Where should I start as far as replacing parts and repairs, based on severity and priority??

Many, Many thanks in advance for any feedback, suggestions and or tips! Jenn