Translation:We did not have enough money for two couches.
"Хватает/не хватает денег на an object of your buying" always works with на.
But You can say "хватает/не хватает денег для покупки двух диванов". Для покупки = for buying" and after that you can place an object of your buying:
для покупки двух диванов.
Pay attention that after the using "для doing smth" you have to use genitive case: для покупки двух диванов.
That's because деньги (plural) is not a subject of this sentence (which is apparent from the fact that денег is not nominative). It is actually a direct object, and objects don't determine the verb forms. This Russian sentence does not actually have an explicit subject, which means that its implicit subject is something like English "it" and hence the verb is neuter & singular. In general, this is quite common in Russian sentences with a "missing"/implicit subject, particularly in those describing a state of something (or things in general):
Мне было холодно. (I was cold.)
Нам повезло. (We were/got lucky.)
У нас не было еды. (We did not have food)
As a result, "нам хватает X"="There is/are enough X for us" always uses singular хватает (in any tense), while X is either singular uncountable or plural countable.
I think you are exactly wrong. Without "enough" you are implying that you don't have the right kind of money to make the purchase (food stamps, for example) but you are omitting the reason. Using "enough" specifies that you don't have a sufficient quantity of money. The difference is, if the price was cut, you could make the purchase but if "enough" is left out, you can't make the purchase no matter what the price.
Your reasoning is logical, but colloquial expressions are not always strictly logical. In my (long) experience as an English native speaker, "we did not have the money for two couches" clearly and unequivocally means we did not have enough money. If we had money but the wrong kind, we would say something different, such as "we don't have the pounds sterling to buy two couches". I think any Canadian, Brit or American would be familiar with this.