Thanks for reaching out!
We checked on the Ghost Square Cross 1.8 bike and, unfortunately, it looks like that shipment has been delayed until July. Additionally, we do not have the ability to send alerts when the product is ready or to pre-order items. Your best bet is to check back on REI.com around that time to see when the bike is available to purchase.
We apologize for not getting back to you sooner! In terms of the difference between the Ghost Square Cross Essential U, the Ghost Square Cross Base U, and the Ghost Square Cross 1.8 are mostly contained in the different components and group sets used. The Base U and 1.8 are very similar with the Base U using a few upgraded components and the 1.8 having a broader range in the crankset. In terms of which size would fit you best, we would recommend a size medium, as the standover height is just under 31" and should fit you well. From what we can see, it looks like the Ghost Square Cross 1.8 we will be carrying in 2021 will come in just below the $800 mark.
Hopefully this helps, thanks!