By "custom built for Sonar" do you mean you bought a system from a company that builds computers for audio work (like Studiocat or ADK) or did you build it yourself?
Or maybe you bought a prebuilt, off the shelf system like a Dell or HP from Best Buy or something.
This is important because if you bought a custom build from a good DAW computer builder (and they knew you were going to be using Sonar) then you are likely good to go (aside from driver/program installation but they would normally do that for you as well AFAIK).
If you built it yourself there will likely be a few windows tweaks you'll want to do, such as ensuring Windows is set to Performance mode and none of your hardware (especially USB/Firewire/Data Drives/the OS/etc aren't going to "go to sleep").
If you bought an off the self system with windows preinstalled you are going to want to remove all the "bloatware" that came with the system or do a totally clean install of Windows. If you are going to uninstall the bloatware (instead of a clean install) then use something like CCleaner or RevoUninstaller to really scrub the bugger. Figure out what ALL the preinstalled programs are and make decisions of what to remove from there. For what you decide to keep make sure you get right into their individual "options" setting to ensure they are not going to take any automatic background actions like searching for updates or autolaunching or anything like that. Also find where to disable "Startup" programs or "Task Scheduler" in windows to make susre there is nothing on the list that might distract your system from doing your audio work.
If there is a wireless network card on the system learn how to disable it so when you are working with audio you can keep it from distracting the system.
From there it's probably best to install and/or update your system drivers and BIOS first (unless the computer was custom built by a DAW builder... in which case don't do that without their go ahead).
Search for and install ALL available Windows "Important" updates (and if you are okay with the optional updates get those installed too... I think Win10 forces you to do this anyway).
Create a system image. So you can return to this clean/first setup state.
Then install the drivers for all your necessary audio devices one at a time creating a manual "Windows Restore Point" each time (that way if something goes weird with an install you can use restore to get back to that point instead of doing a full system image reinstall). Most driver/program installations will create a restore point anyway automatically but I make my own with a descriptive name anyway just in case (and with the name it's easier to know what to revert back to).
Once you have all your audio devices installed start installing your audio programs, starting with your DAW of choice. It should be noted that IF you own previous versions of Sonar (such as X3, X2, X1, etc) and you think you MAY want to get at them to maybe access older projects (in case SPlat doesn't read them properly) you should install those versions in order of release. So X1 first, then X2, then X3 (you can skip versions you don't care about but just make sure you install in sequence). Also remember that some plugins have been removed over the years so if you use those plugins (eg: VC-64 and V-Vocal have been removed) you need to install them from those versions of Sonar. You can use the "Advanced Installation" option to get at just the plugins of each version though so a full install of the program isn't necessary.
It should be noted that the BASIC Sonar installation for each version is actually very small and takes up very little disk space. It is all the content libraries (such as the loops and instrument/synth samples) that take up most of the space. So you can safely install older versions of Sonar itself and just not install the synths and sample libraries (and most of those will come with SPlat anyway). You just gotta figure out what your needs are and what you'll miss if it's not installed.
Create Restore points through out this entire process for each installation.
After you get everything set up, installed and tested/working create another SYSTEM IMAGE (not a restore point but a full image) of your now fully set up and functioning system. You can return to this point at any time by reloading the image or even transfer it over to a new disk drive (in the case of a total system failure).
I am not a tech nor a DAW builder (although I did build my own system and it runs pretty good after over 3 years of kicking the crap out of it).
I have not installed Win10.
These are just general guidelines based on my intertube scourings and my own experiences as a highly motivated DAW user.
Please defer to the advice of proper professionals on all these matters.
Cheers.