Let's see...
First of all: hacking a wii is totally different than a PSP or DS, and very probably different than a PS3 or Xbox. Most brick reports we get here tend to be from people who "assumed" things were the same and used outdated guides on new wii's, and used their own logic when things didn't work out.
Second: the firmware barely matters. Yes, from 4.0 you can use SDHC on the main menu. But that's about the only known advantage. 4.1 did fix a bug in wii sports resort. 4.2 and 4.3 added nothing but antipiracy.
However: though it's not advised to upgrade, downgrading your system menu IS BAD AND UNNECESSARY!!! (as of writing) every wii can be hacked. And in a pretty straightforward way as well. Check
this thread about modmii. It'll get you all the files and give you all the info you'll need.
1. no you DON'T know that "you can flash by USB and DVD (chip)". Unless you have your personal meaning about the word 'flash'. The closest you'll get to flashing anything on a wii is when you update your IOSes and cIOSes (these are pretty much the BIOS of your wii...except that it has more than a single one). Or, if you want to stretch the meaning: install priiloader (which overwrites your system menu).
What you read is most likely about the installation of the homebrew channel. Yes, that'll have to be on SD (or SDHC when on 4.0 or higher). But that installation only has to be done once. Once you have that, you can proceed with IOS and cIOS installation, installation of priiloader and all the other steps that involve hacking (once again: check modmii). Once that is done, you can use the majority of FAT32-formatted USB drives to load all your homebrew or other stuff.
And...flash by DVD? I know there are modchips out there that let you play games from burned DVD's. However, loading by USB simply is superior to DVD loading on all fronts. I suggest you go with that and don't go screwing around opening your wii to end up with lots of frustration, trial-and-error...to get something that STILL loads games slower than from USB.
2. the weight of the games are really big...compared to what? The far majority of wii games don't even fill up a single DVD (4.5 GB). There's about a dozen or so games on a dual layered disc. And those would still be considered "small size" when compared to PS3 and Xbox360 games. But as godreborn said: USB loading is faster than from DVD (heck...it's even faster than from retail DVD).
3. new wiis all come with 4.3. I'd only take note if the wii you're going to buy has gamecube ports or not (the 'family pack' is a bit cheaper because nintendo removed those). But even then: if you're not going to play those games, it's not worth the hassle.
As I said earlier: go with modmii and you'll be fine to get it hacked properly. Not having bootmii as boot2 may sound like a risk, but that's pretty relative. If you want to be safe, install priiloader (yes, this overwrites the system menu...but this once-time-risk pays off with protection from bannerbricks* later). If you want to be
really safe, install neek2o (this basically emulates your entire wii on your SD/USB drive...which means you won't be using the internal wii drive for anything anymore).
*a banner brick is when you install a wad (=a channel on you wii menu) with a faulty banner animation. These wads are pretty rare, but they DO exist. And installing them can cause the menu
like a 250 dollar kit back when DDR4 was Intel only