Easy to install and last.

Scott mentioned, if you lack experience, get an inspection up front so you don't miss something and they can come back when you're done to ensure the buyer will be happy.....home warranties can be worth it in the retail market.Don't tie up money in materials to just sit there, buying an HVAC unit 3 months before it goes in isn't wise, order it, have the supplier hold it, pick up and pay before installation. 

Otherwise, maybe a gentle reminder that they're responsible for the lawn care - and maybe offer to install a timer so they don't have to think about it.  

In addition, you could add small canopy's to the top of the windows for aesthetics and also as a sun block.Through both you will get alignment of the windows, some vegetation or color from the flower beds, and added aesthetics for a relatively cost efficient price.I agree with @Gilbert that earth tones might work really well with this very low rise complex particularly since you're wanting to avoid installing grass.

I do not install travertine in any rentals, however, I do install it in all my Flips.

Installation labor around 350/slab.

But  we're both new at this so that's why I'm here asking.Say we decide on hardwood floors at $11/sq foot installed, and say I find  a deal at $3/sq ft and sell them to him for $9/sq ft and then earn minimum wage for the installation, and we pay the 19% for the hours worked and I collect  $4000 for the completed floor? 

Also if you do find a local window installer make sure they know what there doing, there are few very important steps that alot of people try to skip to save time and money and cost you in the long run. 

@Rex King, you wrote: "We will not install four separate furnaces and each tenant can control their own heat and pay for their own gas".But I am guessing you meant: "We WILL install four separate furnaces and each tenant can control their own heat and pay for their own gas"?

The former is far more common overseas than here in the US and many may be unfamiliar with this window type.I'm guessing that you're after a window treatment you want to install on existing T&T windows... but clarity of nomenclature will help tremendously.If my assumption is correct, are you after wood window blinds or are you considering other options?