I would personally recommend a verified and reputable dealer, or make sure you can try it before buying it.
I tried purchasing a p51 from someone on FPN and the tipping was ground out of existence. The person allowed me to return it, though if they didn't I would have filed a complaint on paypal anyways.
Then I tried getting one from a pen show that actually had proper tipping, the pen looked great, but it wouldn't write when the dealer filled it with ink. He gave me a bogus excuse that p51's required pressure to write.
That's a shame, sorry to hear.
My own experience goes against that. I've bought numerous cheap/bargain 51's and they've all been good. They're very robust pens. I often pop into ebay and if I see one going for less than £20, I'll buy it - even though the last thing I actually need is another pen!
Your mileage may vary, of course.
I'd agree with going for an Aeromatic model, for your first 51.
And sure, try a Wing Sung, if you want (i've had some of those, too) - but, you're going to end up with a 51 at some point, so why not start off with one. Just don't spend hundreds on a rare colour variant and you'll be fine.