Two words: Red Dot
You already have a couple of 9mm's and want a more full size 9mm
Have you also given a thought to Red Dot Optics that Co-Witness with the Iron sights.
Red Dot's are highly recommended and get great reviews.
Just do a youtube search.
If you want something that is a great shooter and something that you "Don't Have"..
Why not look at a Sig Sauer P320 RX (compact or full size)
I have a P320 Compact RX...It was $750.00.
I know that it's just outside of your $700.00 budget but with a Sig Sauer Romeo 1 Red Dot that co-witnesses with Sig's Night Sights setup and direct from Sig Sauer.
I think that it's well worth the extra money!
Without the Red Dot a P320 would be below your budget but I say try something new.
P320 Compact and Full Size:
https://www.youtube.com/watch?v=HLs-qZMyL70
https://www.youtube.com/watch?v=i_RUpjIyCeg
As seen in the 2nd video..
I changed out the "Medium Black Grip" assy. that came with my P320 and put the modular "Trigger Assy" into a "Small Flat Dark Earth" (FDE) Grip assy.
Walther PPS M2 RMSC
This is new for 2018 and ships with a Shield Red Dot
Shipping this month..MSRP $699.00
https://www.youtube.com/watch?v=R3Y0-G0lquM
The Glock 19 MOS (modular Optic System)
Unlike the two above the G19 MOS does not come with a Red Dot.
The slide is cut out and included are different mounting plates.
What I liked about Sig's P320 RX is that their Romeo 1 Red Dot was already on the gun along with Co-witnessing Night Sights.
https://www.youtube.com/watch?v=hPLnwNZFMRY
Whether you take the Red Dot Optics or not all of the above guns are good shooters.
Well at least the P320 & G19, I am not familiar with the Walther.
Mackie244 / Bud