Before you read my opinion. Just know that I am not an investor yet, I only have one house and it's the one I live in. (I have to build my credit up before I apply for a HELOC to fund my first purchase) The only experience I have is that I have remodeled parts of my house. Some things I hired a contractor for and other things I did myself.
With that being said, I understand how you feel about looking at properties and not knowing what needs to be fixed or replaced. It's hard when you are just looking at pictures and sometimes they exclude certain parts of the house so you question if they are hiding a disaster.
First thing you have to do is figure out if you are planning on flipping or renting out the property. There is a big difference. One you are trying to build value and the other you are just trying to provide a comfortable living space.
Let's have a simplified example.
You purchase a property and you know for sure that the bathroom needs a rehab. The last owner decided to just be destructive and wrecked the place. You see a picture and it's just a mess. Broken tile, cabinets hanging off the wall, dry wall holes, etc.
One way to go about it is to just go to a local home depot and have someone check out the bathroom and redesign it for you. Easy. Simple. Then you can focus on something else, possibly your next buy or next steps. But do you want to trust someone else with your money? Some companies are more honest than others but ultimately they are about making money.
The better way to go about it is to do a bit of research.
Estimate the sq ft of the bathroom. Measure your own bathroom if you have to or your parents bathroom. It's just to get a rough estimate in your head.
Assume you have to replace all the tile. Go on home depots website and check prices (materials are the easiest thing to estimate since the price is straight up). Keep in mind that this is a rental, so you don't want the fanciest tile. Your goal is to find a tile that is durable and will hold up for as long as possible, so read the reviews or even go to the store in person. Don't over think the material! Remember it's about making a comfortable home not a fancy one. Then goto a site like: https://www.homeadvisor.com/cost/flooring/install-... that way you can get a median price of how much an installer would cost.
...I want to add that this all depends on the neighborhood also, if you're in a rich A class neighborhood then you better be getting quality that matches, but since you're starting out I'm assuming you will be in the less than 100k range.
From there you repeat the process with the other things. Drywall, paint, toilet, sink, bathtub, windows, etc.
You'll know if you can salvage stuff later, like counter tops and cabinets. You could apply a fresh coat of paint over them without replacing them.
The tricky part is if the electric, plumbing, roof, and foundation. You can estimate the costs the same way but it's harder to know if the roof has a leak in the pictures without setting foot in the property.
As you know some listings will say "needs roof rehab, needs electric work" at least you can prepare for the worst and budget accordingly.
Another method is to just assume those major things are broken and add it to the rehab estimate. After estimating the worst, is it still a good buy? If it is then there is nothing but savings when you find out that instead of a new roof it just needs some shingles replaced.
Hopefully I wasn't rambling and this was helpful. This is how I evaluate, I'm sure there are better ways but you should at least know what materials usually cost and the average labor rate in your area.
