Answer: buying is cheaper almost every time for this, and in your case looks like even after 18 months you are still better off buying - either from the store, or finding another used horn.
How I know: Worked at one of the largest band/orchestra rental stores in the country for 5 years - the commission structure when I was there was such that it behooved us salesmen to sell a used trumpet for $250-350 rather than rent a similarly used one that would get the company about $1,000 over a 3 or 4 year period. Also was an easier sale - much less paperwork. This was obviously not aligned with the best interest of the company, so they took us off of commission after I was there for about 4 years to make us indifferent to the rent vs. buy decision - can't tell you how many people I talked into buying a used student horn because it was clearly better for them (and me). Needless to say, staff turnover increased dramatically when that no-commission decision was made.
Anyway, I digress - I probably would stick with a major brand, even if you go through another channel - Yamaha, Bach, Conn, etc. - the REALLY cheap instruments are often almost un-repairable, and we are talking about kids here - there will be a dent or two, even if you start with a brand new horn. At the store I worked at, you could put any major brand instrument on the same maintenance agreement as came with the rental for $59 per year (at that time - may have changed in the 8 years since I've been there). If you didn't buy it from us, you just had to put it through any necessary repairs first - had to start in "working order".
Edit: Probably better than any maintenance plan is to just teach your kid to take of the horn. Brass in particular are pretty robust instruments, if you take care of them.