1) Is physical attractiveness subjective or objective?
Sorta both. There are things we, as humans, are wired to find attractive, but little nuances are where we all differ from one person to another. If you were to ask 10 different men what their "dream girl" would look like, most of them would have similar features, but they would look like different people...obviously. So, it is broadly objective, but objective once you get down to the nitty gritty details.
2) Is physical attractiveness obtainable for most people?
Tough question. As someone who wants to say, "everyone is beautiful in their own way," something is stopping me. I've seen some ladies who I could never imagine anyone being attracted to, but many find significant others who love them. So, I guess my answer would once again be yes and no. Objectively, no. Not everyone can find a way to be attractive to the masses the way many of today's sex symbols are, but subjectively, yes, seeing as how someone random dude out there may find some chick hot when 99.99% of other dudes would turn away and run, based on looks or her personality.
3) Is physical attractiveness a justifiable basis in choosing a partner?
Yes, but it should not be the only or most important one. It would be unfair to say that you should not take physical attractiveness into consideration, as one, after being in a relationship is expected to participate in sexual activity, which I can imagine to be a pretty gruesome process if you are not attracted to your partner. However, personality should be the most important thing in finding a significant other. Also, if you tend to dig a person's personality, usually, they will start looking more attractive, and vice versa.