When I went to Nepal a couple years back for the first time in 6 years (I stayed for 2 months,too), I enjoyed every moment of it. One of my closest friends' favorite hangout spots turned out to be one of those cheap restaurants (similar to a bhatti) and I had no problem eating sukutis (no aftereffect too). 

It all depends on one's perception and views. 

Nepal is absolutely a great heaven. You just need to board the plane with the mentality that you're returning home. No matter how 'awful' one's one home is, one endears it a lot. 

Few advises - don't drive for a couple weeks. I was literally at the edge of my seats when my brother drove us home from the airport. I'd thought that I'd have no problems adjusting to that aspect but the transition was really tough. But it took me only 2 weeks to get used to everything and I was driving around myself. 

If I were you, I'd try to travel around as much as I could. Don't sit around in home and the neighborhood cherishing what you've missed. I'd never traveled outside Kathmandu prior to my trip to Kathmandu and I traveled across the country extensively. I wish I'd managed to trek around Annapurna region, something I plan on doing before I get married. 

You need not literally watch out on where you eat, per se but I guess its something that relies heavily on your body's mechanism and immunity power. I wouldn't drink tap water though. 

Durbar Marg is a cool hangout spot and if you're looking into dining out, Jhamel (Jhamsikhel, Pulchowk) area is pretty amazing,too. Then there's good old Thamel. The only thing that sucked about my trip was the lack of nightlife because of secuirty rules imposed. There are plenty of decent bars around the Capital too. 

I wouldn't shop around for apparels that you wear everyday. I prefer Jeans that one could buy over here for a decent price that fit. Suits are different items though. Buy as many DVDs (A Dvd for less than a buck, LOST season 1-6 for 15 bucks etc!) and sports apparels (if you're a Sports fanatic that is). 

Eat  plenty of momo but at the same time, eat Daal, Bhaat and Tarkarki every single day (twice), too. Sukeko macha, Gundruk ko achar, Koirala ko Achar (I totally forgot Koirala ko achar before I returned back), [Disallowed String for - castist reference]i Cuisine like Bara, Chatamari, Gwalmari, Sisi ko achar...to go with Thakali khana. Don't forget samosas, chana chatpate and pani pakoras.... 

All in all, Enjoy every bit of it! 

Good luck!