For points 2-5 I think the iPad Air (latest one?) should do the job just fine. I'm still using my old iPad Air 2. But for MS Office use espc Excel it's a pain in the ass. Unless you're doing very simple task using not much formulas.

My personal laptop recently hdd just died, and I'm living on my Ipad 2018. Here's my opinion.

1. Microsoft office (mostly excel to do financial records)
2. Showing sample and photo to customers
3. Web browsing (webpages, fb etc)
4. Watch video (youtube, netflix etc)
5. Bring along the tablet everywhere with her

2 to 5 will be just fine with an Ipad.

But for 1, if your excel requires u to create chart, having lots of formula, then it might be a bit tedious. But the thing is with IPadOS, you can connect it with a bluetooth mouse, so that might help a bit. I'm using a lot of excel to for my finance tracking.

1. Is iPad 7th gen will do the job? I am in dilemma choosing between iPad 7th 128gb or iPad Air 64gb (budget constraint)
Both will do just fine imo.

2. Is Microsoft Office free? Or is there any free apps that can replace the Microsoft Office?
Not sure about this. I'm mainly on the Google sheets, Google docs ecosystem. Yes i install these google ecosystem stuff in my ipad.    so that it syncs between my android phone and Ipad

3. Any sifu here also using iPad to replace a computer? Any experience to share?
Been doing this for like 2 months now. U can travel lighter. But then if you type a lot on your laptop, or you do stuff that requires mouse input for precision, you might need some time to get use to it.

Excel for ios by windows

Subscribe for microsoft 365...your or wife later can access file from your home pc

Just add wireless keyboard/mouse and you are good to go

But you have to know that you can only open one Excel file at a time. If your wife needs to deal with multiple sheets, it will be tedious and slower

@dreamers1016 is right. Perhaps u can have more excel sheets instead of having multiple excel files.

Multitasking on an Ipad still cant beat a windows laptop imo.

You can try WPS Office. Quite reputable, free and on par with MS Office.

Btw, iPadOS is reported to have massive battery issues. You might want to hold on the updates first.

Yes yes, that's what i meant. It'll be quite troublesome to open and close each file each time haha