Hello, I am a low level Singed main, but as a frequent viewer of the well known singed mains, I know quite a lot, I also know their opinions, and I agree with most of them. So, let's begin.
We'll start with a well known(not) fact, his passive is a trap. Mana only helps him in the early and a bit of mid game, afterwords, as long as you do proper management of your abilities, you won't drop in mana that quickly, unless you're in a long winded teamfight. It's best to focus on important items for him, such as Deadman's plate, Rylai's, and Liandry's, instead of mana items as they primarily provide little for him. Yeah, great, bonus health, but do you know what will give him more bonus health? Tank items. :p Silliness aside, His passive primarily works as a means for items that do grant mana, as well as additional stats (ie, Frozen heart, Rod of ages, Righteous Glory, Frozen heart, etc) to be more cost effective. also, I don't want to be that guy who says "Hey dont do tht thats shiit", but Tear is actually regarded as a poor item on singed, although it remains in his recommended. But, you do you shotty. If you can make it work, good on you.
Now.. on to proxy. Level one proxy is kind of null and void now, as the risk you take outways the benefits. With the changes, you can only stay for two waves, and that's only enough to buy you an early pair of boots, you don't even get a level up anymore, due to the longevity of your stay being cut. Though, proxy as a whole hasn't been cut. If you have an opportunity to proxy later on, take it, as it will more than likely be effective.
As it stands, that's the extent of what I know about him for this season. It'd be best to watch people like Chimpso (as WookieJesuz pointed out), or that NA Singed main, I cannot for the life of me remember his name. But, there is at least one well known singed main in each regional server. So, go crazy.
Another thing to add, There are a lot of items that synergize with him, so it's up to you to figure out what works best in what situation, such as, if tear works better in this situation than RoA, I'll build that, or if neither works, rush a different item. If there are no hard CC or devastating spells you need to block, SV will benefit you more than BV, as you get cooldown reduction (ult synergy) and a bonus healing passive (more ult synergy).