Relative Clauses :( buuu

I did too much mistakes in this sentence. I'll give you two separate questions for Polish and Natives:

To Polish:

błedy w cudzysłowiu

Przetłumaczyłem:

Nawet najmniej kosztowny z domów, które widzieliśmy, to więcej niż możemy zapłacić.

na:

Even the least expensive of the houses "which" we have seen is more than we "are able" to pay.

powinno być:

Even the least expensive of the houses (that) we have seen is more than we can pay.

dlaczego nei może być tam which oraz dlaczego nie able to pay?

to Native-English:

"my own" version= incorrectly

Even the least expensive of the houses "which" we have seen is more than we "are able" to pay.

true version=correctly:

Even the least expensive of the houses (that) we have seen is more than we can pay.

Is my version are truly incorrectly?What about which in this case? and able to, Its really never exists?

z tym "which" jest dobrze

to drugie:
to be able to do sth - dotyczy jednorazowo jakiejś sytuacji. Nie stać Cię na dom za 100'000 zł, więc i Cię nie stać na mieszkanie za 70'000 dajmy na to. Ogólnie nie masz za wiele kasy, więc mowisz "can"/"can't".

When I was young, I could run very fast.
I wasn't able to help you yesterday, but today I am.

Z which jest dobrze, ale częściej mówi się that, zwłaszcza nieformalnie.