One of our tenants in one apartment wants us to Power wash the aluminum siding.  

Wash hands!

I havent heard of one in WI that requires more frequent pumping. yes no disposal, and advisable to have a filter on the washing machine discharge as well, and good idea to have them drop enzyme tabs monthly as well, but in all honesty, a good working system will work  20-40 years (average life expectancy) or longer with out any issues. around here a 3 bedroom system replacement (tank and drain field) runs around $5-$6K but a mound with clay soil can easily exceed $20K so lots of variables. 

I would acid wash it to clean it up and seal it with a low luster stone sealer.

I eventually got them out of my house by sprinkling the floor with borax and vacuuming, putting a candle in soapy water and placing it on the floor after lighting (yea, it actually works) and washing everything.  

Was he a good landlord?  

They always gave preference to cosmetic issues like landscaping and power washing instead of issues that are hazardous and code violations.

Then they would need to use a specific primer like a DTM primer, Wash primer, peel bond primer, metal etc.

I also plan to spend a couple grand in pressure washing and painting the house. ?

The walls will need to be washed down with TSP to get all the nastiness off, possibly sealed with original Kilz or other serious primer, and that is best done when vacant.