Tax benefits of turning mowing hobby into a business?

As his "little side job" means you have to declare the income on a Schedule "C" and you may deduct business expenses to arrive at the net profit. The net profit may be subject to social security tax and gets carried to your 1040 which will probably increase your federal tax liability. Your husband mows laws to earn extra money so I assume he wouldn't run it at a loss.

Buying a $25k car with all cash?

Very unrealistic your uncle has saved 25K+ on a salary of 19K. How does he live? If you're asking if it looks suspicious, you already know the answer.

If im leasing a small kitchen inside a Chevron do i need to pay taxes in TX?

You have income, you have to report it. Schedule "C" on your Form 1040. You can deduct your purchases and other expenses against the revenue you received to arrive at your net income. Keep good records and receipts in case you get audited.