Read your lease.Any filters that need regular maintenance/replacement should be specified in your lease.

As I would love to break in to Flipping  and flip this 3bd/ 1b house but  I can't  afford the 10 to  20  it Could cost right now so I  will do  a basic/nice  update replace only the carpet  in living and dining rooms,  No opening of walls ( if Flip kitchen would need to be done, it's a small and weird shaped kitchen) put new Kitchen cabinets back plash tile  floor and maybe re tile tub area in Bathroom and paint.

Should i replace every window?

If there are no comps then you would have to rely on quotes you get from General Building Contractors to give you a cost per square foot to build at that location But without actually building plans designed and stamped by an architect and structural engineer you may not be able to get exact figures but a ball park figure at best.

Its easier to budget for when everything needs to be replaced.  

I threw the stove out and replaced it with a microwave and a undercounter 1/2 refrigerator and hoped whom ever rented it would eat out.  

Our goal for REI is not necessarily income replacement, but more for future planning (retirement is still 30 years away for us). 

You need to calculate your "event horizon" meaning how many years you will hold the property.The 50% expense many use comes from over time when an asset is newly built or recently rehabbed with everything replaced that cost will be low.

It's not the cost of investing the equipment that caused me to hire a professional audio engineer but the time to fiddle with everything.

Currently has 2 tenants, plumbing has been replaced, new counter-tops in both kitchens and new tile.