I'd do flatfelled seams on anything I could and double stitch and zig-zag/serge anything I couldn't. Plus probably french seams any straight seams I couldn't flatfell.
Fabric-wise, I'd go with a heavy shell fabric and a nicer lining fabric, if you want to line it. Like, 12 oz cotton canvas or coating wool and lined with 5 oz linen. Remember: fabric choice is going to affect the properties the coat has when it's built: heat retention, water resistance, breathability, flowy-ness, etc. And please remember to launder your fabrics before you cut them. Toss the whole cut lengths you buy from wherever into the washer on HOT with some detergent and let 'er run, at least once. Then, dryer, Hot, with fabric softener sheets, unless you're planning on hang-drying the finished garment.
Threads... If you're really really concerned with the seams outliving you and your progeny, buy some good silk thread and do those heavy duty seams I was talking about (or Saria's way is fine, too). Otherwise, yeah, Heavy Duty threads are fine.
Good luck.
Oh, and, for pattern: are you just looking for an open-front flowy coat-thing? A
standard kyrtle pattern, extended in length to the calf and split down the front, with no sleeves, might do the job for you. Otherwise/also, check out:
http://www.geocities.com/kaganate/kaftan2.html