If you ride for 5 years(!!!) already then HOW you are not ready to lease the horse yet? I just don't get it.
If you feel comfortable with this particular horse and the owner is ready to lease, then why not? Or indeed you can just look at the ads around and lease or half/lease the horse from someone else with the request of trial.
How many times/week do you ride usually? If just 2-3 and it's your plan then it's better to go with half-lease: it's usually 2-3 rides/week and it's cheaper than the full lease.