I have the opportunity to buy a Glock 20 10mm for $450 brand new from a friend. Seems like a good deal but I would really like to add a red dot. Does anyone know of a good place to have the slide milled or should I go find an after-market milled slide? Or, should I pay $650 for an MOS model and forget the pistol that's $450? TIA

How about a red dot mount plate that fits the rear sight dove tail?

https://velocitytriggers.com/product/glock-red-dot-mount/

$1100 is a good deal for both. See which one you like better and sell the other.

Awesome idea if it doesn't have a problem with shifting !

I don't have a lead on a MOS model. It would be @ retail. I have a lead on a model 20 standard for $450

Mine hasn't moved, it's in there pretty tight. Worth a try, much cheaper than machining the slide and completely reversible if you don't like it.

What dot are you running?

Cheap swampfox I got on sale from optics planet

I just added your prices together for the $1100 amount. Looks like you're spot on for the cost of a new G 20 MOS @ $650.

Get both. Like one better and sell the other.

I had Burdett's mill my G19 Gen 4 slide to fit my holosun and they did a great job, I think they charged $100.

Sign up for a GSSF membership, which enables you to purchase a blue label Glock. I think the membership is $90 for 2 years, so you'll get two blue label coupons.

GT Distributors has a blue lable Glock 20 Gen 5 MOS for $478.

https://vulcanmachinewerks.com/optic-cuts/glock/

My son took his here. They're in Plano. Quality work, but I see their wait times are getting longer.

On a related note, which red dot do y'all prefer and what size dot ?

For EDC, adjustable Trijicon RMR w/ 3.25 moa dot

I guess my phone was spying on me again because I just got an email that Brownell's milled slide for the G20 is in stock and on sale. Stainless steel and uses the Zev patent design. Is this a good option ?

I used the same slide on a P80 9mm build. No issues. Everything works fine. Price is pretty good.

Had multiple done by jagerwerks and their nitride is on par to factory finish. Absolutely fantastic work.
https://jagerwerks.com/trijicon-rmr-for-glock/

Trijicon is widely regarded as the "best" carry red dot. However, keep in mind the MOS Glocks are milled for a Holosun footprint and still require an adapter plate for the RMR.