Start with something simple - a small house with basically sound construction but dated/damaged interior fitout. Learn how to do simple stuff like fixing walls, painting, new kitchen cabinets, laying new tile, replacing plumbing fittings. Keep it simple, low cost and in-line with the market expectations for a good quality house in the area.

Do the numbers and if you can't make at least 20% on your outlay (not the total sale price, but your cash-on-cash return) then keep looking until you find something suitable.

The key challenges are less learning how to lay grout but more in buying effectively, assessing the market your customers are operating in (ie, who is going to buy it, what are their tastes, budget and needs?) and dealing with contractors who will necessarily complete work you cannot do (such as electrical).

Every house will sell with the right price attached. That should be the least of your worries. Worst of all cases is that you are halfway through, and lose time/money/energy/cash to complete the job. Then you've got a half-built house which is unsaleable to everyone but a person who can exploit the pickle you've got yourself in.

I have moved into houses and lived there while upgrading the interior and it has always been personally and financially rewarding, but this is not for everyone.