What watch to get?

Hamilton is a solid choice at this price point.

Khaki is alright and Seiko a solid choice. In that price range you might also want to consider a couple others similar to the Khaki. These just came to mind and remember that straps can be changed.

Archimede
http://www.archimede-watches.com/archimede-watches-collection-pilot-deckwatch-klassik-diver-outdoor-automatic-handwound-chronograph-2824-6498-7750/outdoor-automatic-sport-watch.html

http://www.archimede-watches.com/archimede-watches-collection-pilot-deckwatch-klassik-diver-outdoor-automatic-handwound-chronograph-2824-6498-7750/pilot-watch-automatic-2824-handwound-6498-chronograph-7750-b-watch/pilot-42-h-b-watch-automatic-eta-2825.html

Speedbird
http://www.timefactors.com/speedbird.htm

I believe you take off the VAT for euro purchases. Sometimes these watches can be found slightly used on TZ for less.

You can find Hamilton Jazzmasters for around $600.00 if you keep your eyes open on eBay. I own an Open Heart Jazzmaster with a brown leather strap and love it.

Steven Alan: http://www.stevenalan.com/21-Jewel-Automatic-Watch/59933,default,pd.html?utm_source=GAN&utm_medium=Skimlinks&utm_campaign=Primary&utm_content=Text&utm_term=

The smaller version (pictured on the right) of this Maratac watch: http://www.countycomm.com/maratacpilotmid.html

I've had both saved in my favorites as a future purchase for a while - though I'm leaning toward the Steven Alan, I'm seriously considering the Maratac and replacing the strap with leather.

Really love the look of the watch. Any idea as to the quality of the timepiece itself?

S. Allen is definitely not an in-house movement, (though of the ones you listed, I think only the seiko would be) which is fine, but you may want to do some research and see what kind of movement is in each one of these; you may find that you're paying more for the name than the watch. For example, the seiko 5 uses Seiko's mass-produced 7s26/36 movement, which is their standard, very durable, shock resistant movement. They're inexpensive and durable. 

The Tissot is quartz, which puts it in a different category. It may be more accurate but it's not automatic (this comes down to personal preference). 

 

Just my two cents, but my suggestion is to look at the watch's mechanics as well as the looks. 

That Maratac is good if you're looking for aviator style otherwise I would suggest vintage Omega, vintage Tissot, Sea Gull or Seiko if the budget is $500.

Not to appear condescending, but from the lack of specifics in your initial question I would conclude that you might benefit from learning more about watches in general. My advice would be to browse the buying and selling subforums at watchuseek and timezone for a month or two to see what you can get for your money. This will also help develop your taste, and perhaps prompt you to ask a few questions that you might not have known to ask. 

Without considering the specific models you mentioned, I have to say that I generally like Tissot watches in your price range.  I would probably go with the Le Locle if you can afford it.