My 2¢ as mac user since mastodons roamed the earth and a relentless sketchup user…
Do not go for the macbook air, they are fantastic machines for what they are (we have one) but you need the dedicated high end graphics card of a macbook pro to drive sketchup.
Ram: 8 gig will run sketchup ok, 16 gig is very good, 32 gig a bit better, it’s diminishing returns in terms of experience. Multiple apps and simultaneous background processes are where I notice this.
The i7 is a faster chip but the real world comparisons are not as different as you might think and for some of the older generations of i5 vs i7 you might not notice a difference in daily use, think 1.6 seconds verses 1.8 seconds for an operation.
Hard Drive. This really matters in the grind of daily use. An SSD is significantly faster at accessing your data and that translates directly into less beach balls throughout your day, booting, saving, opening, operating. Go as big as you can, you can never have too much storage 
As noted above this is no longer user swappable which is sad, so now you have to pony up for the big guns from the start, you will thank yourself later.
Graphics card: Apple graphics card choices tend to be well thought out and continue to be of good service for a long time. The 15" has better options, get a dedicated card, not just the built in.
I currently think the best value in a macbook pro IMHO is around 2015, There have not been any major updates since then (unless you count gizmos like the touch bar or taking away all the usefull ports for USB-c, taking away the magsafe charger, or making a terrible “butterfly” keyboard in 2016-17 that has known issues). With 2015 15-inch Macbook Pro you get a solid unibody machine that will run Mojave no problem, has a great array of ports on the side so you don’t have to buy a collection of dongles to connect your externals, has a keyboard that does not break, and is widely available with 16gb ram and a 1TBssd and a decent dedicated graphics card that plays nice with sketchup (AMD Radeon 2048 MB in the 15" only, not the 13"), all for a price that won’t break the bank… Just a thought.