I'm looking to buy a finished product, enhance it and then resell, with or without the original companys logo

Patent law would not prohibit your reselling the article even if covered under a design patent or utility patent. You would not infringe the copyright by reselling the shirt. However, you could not, without permission, make copies of the logo separate from the shirt. The main goal of trademark law is to prevent customer confusion as to the source of the trademarked goods, i.e., am I buying from the company or from someone else? Redecorating the shirt and yet retaining the company logo would likely constitute creating customer confusion. Thus, you would want to remove the logo. Likewise as to customer confusion, if, for example, the shirt itself has a distinctive configuration or shape (such as a feature of the sleeves) that consumers have come to associate with the company logo, then reselling the redecorated shirt might  create customer confusion and thereby infringe upon the trademark. Also, be careful in your advertising not to say anything that might mislead consumers, e.g., regarding the origin of the shirt itself. To avoid trademark dilution if applicable, I would hesitate to, without permission from the company, mention in your advertisement that company as the source of your shirts. Provided the shirt itself does not have the above-mentioned distinctive configuration or shape that consumers have come to associate with the company logo, you are free to remove the trademark (design and words if any) from a shirt you have purchased, to redecorate the shirt, and to then resell the shirt.  

The "first sale doctrine", in the patent context, is known as the "patent exhaustion doctrine." Patent law would not prohibit your reselling the article even if covered under a design patent or utility patent, at least because you have already paid the owner of any such patent when you purchased the article. See https://www.theiplawblog.com/2010/04/articles/patent-law/patent-exhaustion-clarified/. The first sale doctrine also applies to copyrighted works such as any pre-existing logo or design. You would not infringe the copyright by reselling the shirt. However, you could not, without permission, make copies of the logo separate from the shirt. See https://www.turnpikelaw.com/copyright-law-what-is-the-first-sale-doctrine/. As to trademarks, first, if the company logo is considered famous, trademark dilution may apply. Trademark dilution which is a type of infringement may exist if you are tarnishing the reputation of the trademark. Here, for example, if you were to retain the logo and add your own design, your added design should not be such that it tarnishes the trademark's or the company's reputation. In addition, the main goal of trademark law is to prevent customer confusion as to the source of the trademarked goods, i.e., am I buying from the company or from someone else? Redecorating the shirt and yet retaining the company logo would likely constitute creating customer confusion. Thus, you would want to remove the logo. Likewise as to customer confusion, if, for example, the shirt itself has a distinctive configuration or shape (such as a feature of the sleeves) that consumers have come to associate with the company logo, then reselling the redecorated shirt might  create customer confusion and thereby infringe upon the trademark. Also, be careful in your advertising not to say anything that might mislead consumers, e.g., regarding the origin of the shirt itself. See http://seattletrademarklawyer.com/blog/2012/2/28/the-first-sale-doctrine-protects-the-right-to-resell-genuine.html. To avoid trademark dilution if applicable, I would hesitate to, without permission from the company, mention in your advertisement that company as the source of your shirts. Provided the shirt itself does not have the above-mentioned distinctive configuration or shape that consumers have come to associate with the company logo, you are free to remove the trademark (e.g., any logo and any slogan) from a shirt you have purchased, to redecorate the shirt, and to then resell the shirt.