At $15k, don't rule out a 915 transmission car. I think you could get a much nicer 84-86 car with $15k than you can a G50. The upside is that, if you get a nicer one, when the market firms back up again, you're car will rise nicely with it.
I've seen some pretty nice cars with the 915 transmission that I think could be had in your price range.
To be honest, I'd be checking this car out before the ones you listed. No affiliation but you can tell it's been taken care of and meets your criteria except for the G50.
http://www.autotrader.com/fyc/vdp.jsp?ct=p&car_id=252291573&dealer_id=63351971&c ar_year=1986&model=911&make2=&mod_bookmark_id=1725 2261&mod_bookmark_id=17252261&start_year=1981&keyw ordsfyc=&keywordsfyc=&keywordsrep=&keywordsrep=&en gine=&certified=&fuel=&search_type=both&distance=0 &color3=SILVER&color2=WHITE&first_record=26&make=P OR&keywords_display=&color=BLUE&min_price=&increme ntal=All&body_style=COUPE&drive=&default_sort=year DESC&lastExec=1222360576000&max_mileage=&style_fla g=2&sort_type=yearDESC&address=17404&advanced=y&en d_year=1998&pager.offset=25&doors=&transmission=Ma nual&max_price=45000&cardist=1481
So, do you prefer a coupe or a targa? Besides aesthetics (a personal choice), I'm a big fan of coupes because, at this age, the targa tops tend to not fit terribly well and leak.
I'm trying to be sincerely helpful so let me know if I'm being annoying.
Oh, and, if this is your first 911, buy Pete Zimmermann's book. This'll be my first Carrera and his book has made me a much more informed buyer.