I've lived over here (Asia) for the past 20 years and traveled all over - most of the time with cameras like the Oly 3040 and then onto bridge cameras before getting into DSLRs a few years back.
1. you are traveling in the dry season for this particular region so there is less concern re. heavy downpours, but it will be hot and humid. If you are in air-conditioned accommodation beware then when you first venture out in the morning condensation is a big issue so keep your camera and lenses in vacuum packs until they have warmed up. If you do have any condensation it'll soon evaporate once the temperatures equalise. I don't think WR is a big issue here - take a wet weather cover that folds away into a very small package if you are that concerned - I've never had an issue since when it is raining here you are inside not out (you better hope) !
2. You absolutely want to have a back-up camera you are comfortable with if this is going to be a long trip, e.g 4 weeks + and not the standard 2 weeker (where I'd take your K7 and a good P&S). At worst a very very good bridge, personally in your situation where you have a budget, but not a deep one to change your whole kit, I'd ditch the K7 and go for 2 x Kr (think what you are spending on the trip and you can easily re-sell the two Krs and then get your K5 when you return). Smaller, lighter, better overall performance, though no WR (not essential IMHO) and will get you noticed less - make sure whatever you carry is kept in front of you or well locked away if behind you, pick-pockets (inc. gangs of begging children) and blades/knives to slit open bags are endemic even if you are unlikely to come across them unless you are unlucky or too flashy.
3. The question of focal lengths. Wide, well 17 is often not wide enough but without packing a 10-20/10-24/12-24 then you should work on your stitching skills and buy a good stitching program if you don't have one. My best landscape lens is a 28mm - for you I'd say the 31 and stitch. The 17-50 / 18-50 lens is a work horse when in towns and market places so I'd add the Tamron 17-50 to your kit and sell off the Sigma and Pentax. The Tamron is a superb lens IQ wise. Don't ignore length. You can get some superb shots of people working in fields etc. and again if either you or they are shy, with a long lens, and to keep it light and for very good IQ I'd say the 55-300, reverse the hood and this is really very very small and light for a lens hitting 300mm (though the IQ for this lens is better at 240-260mm and then cropping to 300mm FoV than actually shooting at 300mm where it softens up just a little in comparison). You absolutely must take a low light lens - there isn't much in the way of street lighting at night in these towns and interiors are usually very poorly lit. The K50/1.2 would make it into my bag every day of the week (for cool low DoF shots too). You might want to consider buying a Raynox 150 too as an add-on macro lens, to any of your lenses. Just a few dollars but superb IQ for the money - and there are lots of interesting insects, flowers and plant life over there.
4. Never, ever, ever, ever take just one high capacity SD card (not that you can't buy them over there too though). If anything happens (and it does happen - just listen to the stories, I've only had a corrupted card once but what happens if you lose some gear, have it stolen or damaged, do you really want to take the chance of coming back only with memories and no photos) ? I use multiples of 8 GB (6 cards) and one card is more than enough for one day, but up that to a 16GB if you wish. Also devise a system for marking them, and if you are not going to upload the shot each night, then check and delete. If you are going for a month then you need to have netbook or other form of storage (now we are entering a whole new thread area - check previous topics on PF for storage and uploading on trips) or more SD cards (much cheaper and much easier to carry than a notebook and can be sold off when you return if you don't need so many as say 10-12 cards when you return).
5. From your current kit the FA31 Ltd is a must. No way you can leave the IQ quality of that lens behind on trip like this, I'd be very tempted to say the same about the FA77 Ltd but then I use primes most of the time so take that advice with a pinch of salt if you are keen on your zooms. I could make three kits from your gear and potential purchases - without going to extremes (price-wise) :
Zoom Kit A. : Tamron 17-50. Pentax 55-300. K50/1.2
Prime Kit B : 15 Ltd. 31 Ltd. K50/1.2, 77 Ltd. 55-300.
Mixed Kit C (existing gear but spending your money on 2 x Kr after selling off your K7) : Sigma DC 18-50mm, 31 Ltd, K50/1.2 and 55-200.
Last edited by Frogfish; 08-11-2012 at 02:16 AM.