typically you need a couple dblades before wit's end, but yea, wit's -> phage is pretty tight. wriggle's first also works, having some lifesteal drastically improves your ability to solo push while your team does other ****. I tend to not finish my mallet immediately, but I also make a point of taking red buffs at that point in the game. so usually I'm looking at a midgame build that's like, mercs/ninja tabi/zerker's + 2 dblades + wit's end + phage + madred's razor, then getting bloodrazor and mallet as necessary. Then I get a GA usually, but sometimes banshee's veil, atma's impaler, Quicksilver Sash, or Randuin's Omen feels like a better defensive choice, they're all kosher. Last item I'm obsessed with bloodthirster or starks (if I have a 3+ autoattacker team) so that I can 1v1 people with that lifesteal. whole point is that Teemo's a tanky DPS who just so happens to be ranged, he's not a ranged DPS as his kit is all about base damages and not scaling (he basically has no standard offensive scaling, AP ratios are meh and his only standard AD bonus is the AS from his passive, which is unreliable at best).Well, I primarily love him for how much utility he has, but he can really mess people up.
I usually just go boots+pots into boots2+witsend for pushpushpushpush and so I don't die immediately when a mage ganks me. Other ganks don't even need to be worried about. I have my runes and masteries set up for some magic pen and uber tanky thanks to some insight from Mogwai.
I usually go mallet after witsend so I can roam gank after tanking top. Mallet is ****ing ridic on teemo with that sprinting nonsense, allowing you to single out an opponent easily.
After those 3 items, I think my build could use a lot of improvement, but I usually go bloodrazor or wriggles for tankyness or extra deeps. I think I should change it based on if I'm going to be split pushing towers or sticking around for teamfights, which would mean I have to branch between damage and further utility. After that 4th item I usually end up building something tanky. If they have more than one AD carry (ranged + jungler or something), I figure I can only blind one of them, so I might as well get a frozen heart. Lets me shroom it up more too. Then void staff. Because magic damage.
*shrug*
After the first 4 items, I really feel like I'm at a good balance of utility and damage. My role is never to be a straight carry, but rather to gently guide my team to victory through massive map control and being able to single out opponents, causing inopportune teamfights for the enemy, where my utility is further used to shut down opponent AD and live long enough to eat the enemy support.