I typically don’t swing a two hander for stripers. My friends and I use 16 - 18 foot boats or kayaks in the spring to target stipers in estuaries, salt rivers and salt ponds. We usually are not wade fishing because we want the flexibility to move around as the tide and fish move. Some coves will be a ghost town and adjacent ones, or one across the river, will be on fire with bait and bass, so mobility opens up a lot more opportunities. Because we're in boats, a single hand flyrod is the rod of choice.
We're using 9 ft 7 to 9 wt single hand rods, depending on size of fish and wind. Often times the Rhode Island spring wind is nasty and a 9 wt is even a struggle, but typically an 8 wt works most of the time. So, a 6 wt two hand rod would work for wade fishing where you’ll expect schoolies in the 18 to 26 inch range.
The few times I've gone wade fishing with a two hand rod for stripers, I used a 7 wt Beulah Platinum 10’8”switch rod (don’t own a 6 wt). If you are only going to buy one spey rod (good luck with that, btw!) then a 6 wt in the 12 - 13 ft range would give you a bit more flexibilty to target trout, landlocks, and stripers and would be better for windy conditions than a 5 wt. If you were looking to fish just trout and landlocks then the 5 wt would be a good choice – either a stitch or a short spey rod.
I think a 7 wt spey would be a bit much for the trout and landlocks you mentioned – unless you’re lucky enough to hook the big boys. We routinely hit stripers in RI in the 38+ inch range, which may be the case in Maine too. If that’s so, then you’ll have fun with the 12.5 – 13 ft 6 wt spey and the bigger bass! In my opinion, a 5 wt spey would be under matched for these bigger stipers, especially in faster currents / stronger tides. I favor more rod so I can land stipers faster and avoid overtiring the fish.
There are a lot of rod brands to choose from. I’d look in the Spey Pages Classifieds for a good used one. The sellers are usually helpful with info about the rods they are selling. I have experience with Echo TR and SR, Beulah Platinum, Loomis IMX Pro, TFO
Deer Creek, Redington, Anglers Roost, and Sage rods. Some I like more than others, and certain rods just get under your skin and they become your “go to rod”. I’ve never casted an Anderson, but others on Spey Pages love them. Maybe your friend will loan you his Anderson to try on some Maine spring stripers. Try to find a fly shop where they will let you try some demo rods or go a Spey Clave where there’s many rods to test cast. Hope this helped some. Maybe other Northeast striper fisherman will chime in too. Once you get used to the two hand casting you'll probably find that one rod really doesn't cut it. Good luck and have fun!