The tenants will either keep the shower clean or not, regardless of the material. 

True freedom isn't buying 5-6 houses and hoping for the best, it's a combination of suffering(by eliminating expenses, material items, immediate gratification areas) and betting on quality growth(delayed gratification, asset accumulation, perseveration, etc.) and managing the tail and head winds that come with this. 

Also, if there's any resource/materials/books out there you can recommend,  that would be great.

At this age, I am not trying to become fabulously wealthy; my wife and I now value experiences far more than the material things.

If your plan is to use the card to help advance the material purchasing while waiting for draws, thats a valid reason to use a credit card, but I see too many inexperienced investors get crushed thinking they can use a credit card to cover the improvements or even the downpayment and I urge you to stay clear of this strategy.

With a shell, depending on a few things the materials alone can cost 100k before even  bathrooms or windows in.

An agent, wholesale mortgage broker, building material vendors. 

If you're using contractors and getting multiple quotes, they typically can buy material at better prices so I'd let them.

I know that @J Scott wrote a pretty good book on the process though, (just finished it this weekend), and I would recommend starting with that.If you purchase the book(s) from BiggerPockets (as opposed to Amazon), they come with a bunch of digital files to supplement the material. 

I can build the A-frame for $53k in materials and the shed roof cabin for $41k.