Runes are too offensive. You can probably take Scaling Health Yellows and Magic Resist Blues. Moreover, you can put some extra AP in Blue but just flat one and if needed you can use 6 AP and 3 Mana Regeneration Blues because the Blue Mana Regeneration Runes are more effective than the yellow ones.
For starting items you can just purchase two Health Potions and these are more of a preference than Mana Potions as you can always play passive if being forced to but you cannot afford not having sustain in Health Regeneration.
Rod of Ages is kinda meh because it denies your early game presence. Champions like Kassadin can scale with this item but Heimer would benefit more from Athene's Unholy Grail against AP followed by Rabadon's Deathcap + Void Staff or Morellonomicon against AD and other certain matchups followed by Zhonya's Hourglass or the standard path.
I think many players even avoid building one Mana Regeneration item but I'm too unexperienced in playing Heimer to make a statement about this.

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Constructive criticism is highly welcomed, as mentioned this was my first guide and it was more of a way for me to get my footing a little in writing them, i feel like my overall build is pretty decent, but i also feel that i am lacking a lot in actually giving information on my play style. This is also definitely not the final version as i intend to figure out how to not only explain myself a little better but also how to make my guide look much more than just a wall of text.