Tire manufacture date is a stamped four digit number. First two numbers will be between 01 and 52 to denote the week of manufacture. Second two numbers are year of manufacture.
For testing spring rate you can aim to get the same effect as in this image.
By making some hard wood chocks that go through the spring/coils. Secure firmly. I’ve used jumbo zip times through holes I drill. Take great care and on a liability note I would not suggest testing on public roads. Leave the fronts std and untouched and chock the rear. Test, a short drive will reveal much. This is not a permanent solution. There are online spring calcs that will help you figure out the rate of the spring without the chock and then just redo the calc subtracting the number of coils you think were blocked out. I’d say you prob started with a 200lb spring as std and 400lb - 500lb will work best. I’ve used https://www.dfaulknersprings.com/ in England for custom springs and Kings in Australia. I’ve not played with L700 springs but I have done so with M100 3cyl front springs which from a casual glance seem about the same and were about 140-150lb per inch. Cost wise you may, as you are aware, need to find springs from something else with the same diameter and end types.
For tires, in comp stuff or semi slicks I am using Nitto NT01. They are mid range price wise and are really well manufactured. The last set I fitted balanced up with one needing no weights and 20gm total being the most on the heaviest. I also use Nankang AR1s which are the best value semi slick for short club events. Not as well made as the Nitto and took 50gm of weight at least per wheel. The treadwear value for these are 100 and 80 respectively. They are not road tires really. For the road a TW of 180 is not going to last long enough for most people’s expectation. On my WRX I run Kumho V720 which are a TW of 200. My preferred road tires are Nitto 820 with a TW of 300. They are quite, well made and in my sizes reasonably priced. Other brands I’ve had success with are Hankook, Yokohama, BFG, the Kumhos and some of the other older established brands. Check reviews on https://www.tirerack.com/content/tirerack/desktop/en/homepage.html to get an idea.