1. Esp for those with tank at outdoor/balcony, how do u handle the sunlight problem? UV?
Can't advice much as don't have this kind of setup. Maybe try blocking the light as this is one source for algae to grow. Other will be keeping a "clean" water by using DI water, less feeding, less bio load, etc.
2. Is Chengnai a good choice given mine will likely get some rain now and then?
Yes, strongly recommend.
3. Am thinking of 3.5'x2'x2' with external overflow. Possible to go braceless?
Think yes but the tank maker must be very good. If not wrong, aquamedic tank is braceless.
4. And lastly, any good/reliable/reasonable $$ tank maker to recommend??
Subjective to good but you can try to check with Ah heng from iwarna, han teck and yonghua(can go to arowana website to look for them).