Using old iMac as a monitor connected to Mac Studio

Alas, no. You are referring to Target Display Mode which Apple describes in Use your iMac as a display with target display mode.

You will notice that this is an old and outdated functionally and which requires the monitor iMac (the document specifies which models qualify from 2009 to mid-2014) to have macOS 10.13 High Sierra or earlier and "the other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed".

Therefore your potential new Mac Studio will be far too recent to allow this.

Grafiker37 wrote:

Thanks for the info. Is this the adapter you mean?

Yes, that looks like the Apple Thunderbolt 3 to 2 adapter.

Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple

About the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple Support

Grafiker37 wrote:

Oh thats a shame. What about the Apple Thunderbolt Display 27"? will that work?

Yes, you can use an 27" Apple Thunderbolt Display or 27" LED Cinema Display with a Mac Studio – but you need to make sure you properly identify the display. Just matching physical connectors (USB-C, Mini DisplayPort) isn't enough – you need to match the adapter to the type of signal (Thunderbolt or DisplayPort) the monitor needs.

• The Apple Thunderbolt Display (27-inch) (model A1407) has a hydra cable with TWO heads: Thunderbolt (old-style, with mDP connector) and MagSafe. To connect it to a modern Mac, you would use an Apple Thunderbolt 3-to-2 adapter. If the "hydra" cable is bad or flaky, you could try running a Thunderbolt 1/2 cable from the TB2 side of the adapter to the TB daisy-chaining port on the display, bypassing the "hydra" cable.
• The LED Cinema Display (27-inch) (model A1316) has a hydra cable with THREE heads. You would connect the mDP head to a third-party USB-C (male) to mDP (female) adapter (you can find several brands on Amazon) and also connect the USB head so that you could control brightness.

an additional ThunderBolt-3 <-> Thunderbolt-2 adapter cost an additional US$50.

an additional ThunderBolt-2 cable to hang a second ThunderBolt-2 display off the first costs about US$80. cheapness suggests an additional adapter, NOT another cable.

I believe that you can plug the Thunderbolt "head" of the second display's "hydra" cable into the Thunderbolt daisy-chaining port of the first display. If you can do this, you'll only need one TB3-to-TB2 adapter.

This assumes that both displays have "hydra" cables that are in working order. Should either "hydra" cable be bad (forcing you to use one or two bypass cables), you will not be able to daisy-chain the displays, and you will need two Apple TB3-to-TB2 adapters.