I've made the hike. At the risk of stating the obvious, the level of difficulty depends on a lot of things: Primarily your level of fitness and the heat/humidity at the time. For reference, I last made the hike as a slightly-overweight 54 year old man. Challenging, but not overly difficult. The good news is that, if it's too strenuous, you can always turn back and walk down the hill back to Friar's. Gravity will help. Although the path is clear, it's rocky and sometimes a "bushy." Use caution.
Regarding chair rental, I'm not sure why you'd spend money to rent a chair at Friar's, only to abandon the chair you paid for to spend time at Happy.
While there is parking closer, it's in a clearly-marked "NO PARKING" zone, so I would advise against that.
And, yes, bring water. There are no services at Happy Bay.
