Honestly, it doesn't sound natural at all. No one speaks that way in real life. I would suggest: He was well-groomed, but I got the impression that he doesn't spend his money on expensive clothes.

No, it is not a sentence. You cannot take two independent clauses and link them together by a comma. What you have is two sentences, not one. However, each of your sentences is correct. If you change the comma to a period, you have two acceptable sentences. The first of your sentences ("well groomed he was") uses an odd, but not wrong, word order. The most natural order is "he was well groomed". If you want to veer in a poetic direction, you could say "well groomed was he". If you wish to combine your two sentences into one, you have two choices. Either use a conjunction to unite two independent clauses, or transform one of the independent clauses into a dependent one. Here's an example using the second approach. I'll transform the first sentence into an adjective clause. To make its logic clear, I'll make sure the adjective clause is directly next to the noun, "he", that it modifies: My gut feeling was that, though well groomed, he didn’t buy expensive clothes or famous brands." note: "or" is more logical than "and"

It's a sentence that could give rise to great confusion. Instead, you could say: 'well groomed that he was, my gut feeling was that....' All the best!

In my opinion there are two main problems with this sentence. 1. THAT is the wrong preposition; it should be THOUGH 2. The order of the last two clauses, while not necessarily incorrect, is very awkard and unnatural. Maybe: Well-though he was, my gut feeling was that he didn't buy expensive clothes and famous brands.

Pleasure!