Personally (and this is controversial but I think it's the best bang for the buck), I would sell all CoEs and TLs in the lowlands (which cost you 148.5 bil) and put up spy def towers (which will cost you 138.6 bil). Over all cost is 287.1 bil.
Your final stats would be 1249710/1195462/1109480/2697752
You'll lose 2 mil combined attack and defense. And your plunder and AB will go down.
Alternatively, you could buy 7 more HF lands and put total of 8 lvl1 towers there. That will cost 185 bil for the land and 400 bil for the towers. Total of 585 bil but that extra is the price you pay for keeping your atk and def stats.
As for plunder curve effects, that's the controversial part. I think HF and HL towers cut far more deeply into your plunder than lowland towers that give equivalent stats.
Also later on if you want to get rid of your towers, you'll have no qualms selling off lowland towers whereas dropping HF/HL towers will hurt, especially when you also have to pay that 175 bil or more to get lvl3 T5/T6s back.