IME a single abscess will rarely cause lameness for a long period of time. You may have a case where it pops, horse feels better, then it closes up before it can fully drain and it recurs, but for two+ months? Very suspicious.
Trials can be very expensive- I lost nearly $1k on a horse I took home on trial that I really thought was the one. Board, PPE, feed and supplements to match her current diet, and paying someone to haul her back to the owner all added up quickly.
If you're set on this horse, have the PPE done before you bring her home on trial.
“The horse is a mirror to your soul. Sometimes you might not like what you see. Sometimes you will.” - Buck Brannaman
"Nothing forced can ever be beautiful." - Xenophon