Threats to Heimerdinger with this build
|Syndra||Riperoni, Alt F4, ff @ 20. This match up is horrible she can pick up your turrets and chuck them at you (yes even the ulted turret), she can poke without taking damage, and worst of all she is point and click for ult and it she is easy enough to play for the first time well enough to beat a Heimerdinger in lane. NaCl aside the counter play is as follows, run MR glyphs and flat, scaling, or regen hp seals. Build abyssal sceptor first item, and merc treads for boots. Play passive so laning phase does turn into a feed fest and have your jungler help getting her off of the tower. My number 1 tip is to ban her or if you don't have a ban, plead for one, during draft.|
As for the the late game items (only if you are full build), if you have over 500 buy an elixir. If you have thousands of extra gold, sell boots buy ZZ'Rot, place portal, sell ZZ'Rot, and buy boots back. Repeat this process for Banner of Command too.
Flash is a must as Heimerdinger and that spell should be ran on the F key.
As for the second spell, as a mid laner, exhaust and ignite are the best choice. Ignite is great consider it does true damage and reduces the effectiveness of healing, so if they engage after they used a potion it will be give the enemy less health. Exhaust is great for ganks and when the enemy laner is melee. As for ganks you will keep the enemies in turret range and exhaust the bigger threat. This will be great to reduce their damage output, slow so they stay in turret range, and reduce their MR so they take more damage.
Mid vs. Top
This is something I am currently trying to find out at the moment. (In regards to mid) With assassins, especially now, being so strong, they can kill you and come out with little to no damage due to their swiftness. As for top champions tend to be tanky and currently tank are very strong also. The reason the tanks top are an issue is because MR is a stat that works great in tank builds and with champions like Darius they can out damage you after they get Spirit Vasage.