このバックパックはよさそうなので、買うつもりです。
'Because this backpack looks good, I intend to buy it.'
Following ~sou there is a 'na' placed before the 'node'. Why is this the case? Is it because of the ~sou implemented in the sentence? Furthermore, would the sentence still be grammatically correct even if the 'na' was not there?
「このバックパックはよさそうなので、買{か}うつもりです。」
「ので」 can only attach to the 連体形{れんたいけい} ("attributive form") of a conjugated word. It simply cannot attach to any other conjugated form by "rules".
「そうな」 is the 連体形 of the auxiliary verb 「そうだ」; therefore, that is the only correct form that can directly precede 「ので」.
Thus, you cannot ever say 「そうだので」、「そうだろので」、「そうでので」、「そうにので」 or 「そうならので」.
Furthermore, would the sentence still be grammatically correct even if the 'na' was not there?
No, it never would. 「そうので」 is simply ungrammatical because, as I stated above, 「そう」 is not the 連体形 of a word.
I'm assuming you know the meaning of ので and what nominals and the extended predicate are.
よさそう is a nominal (名詞、な形容詞、etc.). All nominals take な before the の of the extended predicate (e.g. それなんです、分かるんだよ、高いんです）.
ので actually uses the の of the extended predicate (combined with で、which here is not the particle but rather is the gerund of the copula だ・です).
In short, the rule is nominal+な+extended predicate.
