Year old PLA should be perfectly fine. I have PLA that’s been unbagged for 3-4 years and prints fine.
BLTouch isn’t supported by Lulzbot on the Taz5. You can go with Drunken Octopus firmware and get it working though, or just build your own firmware.
I honestly wouldn’t put money into an older Taz unless you’ve got a big backlog of 2.85mm filament. Otherwise? Just get a modern CoreXY that’ll be 10x faster and much higher quality output.
I’m sure it depends on local humidity. Some people claim they can detect filament changes in only a day.
The BLTouch system and new print head allowed me to keep enjoying the much lather capacity of my Taz Pro, so it was worth it for me. There are also some benefits of 2.85mm filament over the smaller stuff, but that’s another debate.
I can print PLA mostly fine on my TAZ 5 but your bed leveling is very important. ABS requires about .25mm higher on the TAZ 5 Z axis adjustment screw than what I can print w/ PLA. Also, since you’re printing ABS, swapping from ABS to PLA is VERY difficult unless you thoroughly clean your nozzle and the inside paths. I use a filament cleaner and push stuff thru and do a few cold pulls until my cold pulls have zero leftover ABS. A tiny lil spec of ABS will clog the nozzle and ruin your PLA experience.
When you load your filament and extrude it, you should see the filament fall straight down out the nozzle and create the nice coiled tower. If the filament juts out to the side while extruding, the nozzle has a clog still. I found the easiest/fastest way is to swap the nozzle tip and use one for PLA only, and the other for ABS and can’t hurt to just use one nozzle for each filament type.
Retractions will kill your print if you have tons of retractions occurring. It will mushroom the filament eventually and make a plug so it can’t extrude anymore.
If you have everything else squared away, dialing in your temp vs. speed is about all that’s left.
Also on the note of the magnetic bed on a TAZ 5, you CAN do this. Lulzbot support even said this on a post where it’s just the bed corners that are the issue. I’ve since modified the stock modular bed corners, picked up the modular bed and Octograb and it’s on my TAZ 5 “now”. Technically I have the old build plate on there now but that’s just cuz Lulzbot sent a bad modular heater bed but it does fit/work 
The new heater will be here Thursday so I can re-assemble that bit but as long as it actually is at 100C when it’s set to 100C and not actually 80*C (20% discrepancy from actual vs. setting) then I’ll be good. The temp discrepancy has nothing to do with the modular bed on the TAZ 5 either since I can swap my good modular bed/Octograb from my TAZ Pro onto my TAZ 5 and it works perfectly and the failed one did the same on the TAZ Pro so it was the modular heater nothin else.
The TAZ 5 with that Octograb looks nice and def works very well! Use a dial caliper mount on the toolhead and that print bed should be VERY level and not much need for a BLTouch as is 