The new browser, you "might" get because the Metro UI version of Internet Explorer is gone in Windows 10 and so is Internet Explorer (finally) in the latest build that I just installed on my Pro 3 (build 10166).
That would not be 'firm' though. Cortana could be added but limited. Some of the features would take up too much storage to add and the Surface RT and Surface 2 are pretty humble in that category. The Weather, Mail, and Skype have been redone completely
and would make sense to just carry them over along with the updated version of the Store.
The Metro UI versions of Word, Excel, and PowerPoint are in the Windows 10 store and are the "Mobile" versions (very limited in functions) and are just like the ones for Windows Phones, Apple iOS devices, and now Android. That would be redundant for the
RT models because they already have a Microsoft Office suite + Outlook pre-installed.
Now, the Office could "could" be upgraded to the Office 2016 model.
But, that would be a lot of work to put into two tablets that are phased out but supported via updates for about seven years.
I have the Surface RT as well and it mostly stays parked in a messenger bag and brought out about once a month to run updates and to update some report files I use on it.
Probably best to get a Surface 3 if you want to stay low budget and I do believe that the keyboard for the ARM tablets match up with the size of the Surface 3. You can check "Surface Accessories" in the Microsoft Online store to verify that with the keyboards.
Do not expect much for the ARM based tablets.