I've never seen people selling used test kits, unless you were talking about used tanks?
What is probably most important is ammonia and nitrite. Don't worry about pH, Hardness or Nitrate tests. You don't need them.
Single test kits cost around $8-10 brand new, from what I've seen over there (Petco online is selling them for around that mark). So maybe offering to do some extra chores or grovelling at the feet of your parents might help you put some money together.
Test kits are an invaluable tool for anyone with a fish tank, particularly for someone like you who's not sure on cycling and has just added new stock. You can't really gauge the quality of your water visually, so these provide some assurances as to what is going on.
If your pleco was unhealthy to begin with (coming from such bad conditions as you mentioned), than you want your water quality to be as perfect as possible as its immune system is likely under a lot of stress, leaving it suspectible to disease.
Poor water quality will also affect your bettas' health, and with several un-quarantined livestock in there, something like ich or columnaris could possibly break out.
Hope that helps some.