I just went through this myself as I got my Mac mini M2 Pro just last week... and I've kinda hit a roadblock storage wise. I personally think hard drive speed is THE most important thing anymore - especially if you work with large files like video files (plus, I'm just impatient, I also need to copy files super fast to move from the office to my home quickly). I even paid DOUBLE for the Mac mini M2 Pro over the base model because the drive in that can read/write roughly TWICE as fast as the base model can (unless you get base model with larger drive, but then once you add the ram I want and such I was getting too close to the M2 Pro Price again so I just got that – with base 512GB drive, planning on using an external drive when I need more space).
BUT, I did not know a couple things when I made that decision - in order to get a SUPER BLAZING FAST external drive (just to get one that is as fast as the internal drive is, that's all I want), that means I can't even use a plain old SATA SSD but need a M.2 size NVMe SSD)...
Problem with those is even the cheapest M.2 external drive enclosure with fancy/new-age Thunderbolt 4 (or 3 even) and/or USB 4 –for 40gbps read/write– is like $130 by itself (more like $160 for the one I want) + whatever M.2 drive you put in that enclosure (best balance of price, capacity, TBW longevity, DRAM and all that good stuff you want in a M.2 drive was $160 for a Samsung 2TB... and even that one is one generation back...)... so that's $320 right there... really not too far off from the $400 you were initially balking at (which IS pricy, I agree... but now doesn't look as bad to me...).
The second thing is Apple screwed us all with NOT using the USB standard from 2016? which most other modern nice-ish computers have been using for years: USB 3.2 Gen 2 2x2 (the 2x2 is the important part). That standard can do 20GBPS which is nearly what a new Mac mini can do with its internal drive and would probably be fine for ANY possible needs... and those external enclosures are as cheap as $40-something... but no, can't use that on any Apple computer so the only options are a ~$25 10GBPS cheap-o enclosure (too slow IMO), or the ~$160 ones - NO in between.
Anyway, the above might be overkill, but when I get a new computer I literally plan on it lasting me for like 8+ years old (the 2023 Mac mini I got replaced a 2012... I got 10+ years out of that one!!!) so I try to get the fastest of everything I can afford so it'll still be usable all those years in the future.