Hi Para, there are many ways of expressing your sentence. The one that is closest to your example is probably the following.
1.Even if we invested in education on the largest scale possible, it wouldn't be excessive.
2.Even if we invest in education on the largest scale possible, it won't be excessive.
Note that in such a sentence, both the conditional form (No. 1: investED...... wouldn't be), and the present-future form (No.2: invest...... WON'T be) would work.
Hope this helps,