Can I open an HSA now, before Apr 15th to pay medical expenses for 2017? My wife has Obamacare aged 62, I am 69 with medicare. This year she will meet her $6600 ded.

Because you are covered by Medicare, you yourself are ineligible to make a contribution to your own HSA.  If your wife's insurance in 2017 was a qualifying High Deductible Health Plan, she is not a full-time student, cannot be claimed as a dependent on someone else's tax return, and has no other, disqualifying health coverage, your wife can make an HSA contribution.

One can only use funds from the HSA to pay for medical expenses that were incurred after the establishment of of the HSA.  If your wife presently had no HSA in 2017, funds from an HSA established in 2018 cannot be used to pay for medical expenses incurred in 2017.