Regarding thread weights, you can go by comparing sizes and width, and it doesn't have to be very scientific. You need a rough idea of sew all weight polyester, equivalent mercerised cotton and top stitch weights. They tend to differ a bit from brand to brand, and even within the same brand there tend to be several threads to choose from for much the same purpose. There's two strand thread, three strand thread, thread with more or less fuzz, some are with out. Having a bunch of spools to choose from is luxury. The only thing about old thread is if it's stored bad, it can go weak (just make a snap test between two hands) and even get dirty sometimes. A bag of old threads are usally like new, nylon and polyester last "for ever".
Finding a matching needle to the thread is relatively easy, there is generally a range (a few sizes) that work well with the thread, and you can adjust it to the fabric.