I let my old large screen Eagle go with a canoe I sold. I planned on getting a smaller new lighter weight lower power detector.  I purchased a humming bird that was the store demo unit.  I do not know if the problems are because as a demo unit it ran for weeks with the transducer out of the water or if the unit just does not do well.  It loses the bottom all the time and if it is run in auto sensitivity it can’t find the bottom. If the sensitivity is high enough to see the thermal it thinks it is the bottom.  It is lighter and uses less power.  So don’t buy a demo unit.
As far as batteries go the lithium are still a good choice.  There is a rechargeable that is an equal as far as weight and will run your detector as long on a full charge. Here is a link to info on something I am going to try this year.
(You need to Login or Register to view media files and links)
I am doing some testing on a Hook-4 detector.  Looks like the above battery setups will run the unit for 11 to 15 hours. It draws a little more than 2 times as much as the hummingbird. It is heavy with a lot of the weight in a 30 ft transducer cable.

Not many gray scale detectors.  My old detector was a Eagle Mark and it was a good detector. Check out the Lowrance Mark-5X  Pro.
I found this review but I don’t know when it was written. Some of the detectors may not be available now.
(You need to Login or Register to view media files and links)

Maybe you could find a gently used one or never used  grey scale on CRAIGS list?

You're in Minn right?

These areas have lists. Might be worth a look-see?

Bemidji
Brainerd
Duluth / superior
Mankato
Minneapolis / st paul
Rochester
southwest MN
st cloud

Not much many being manufactured any more but some available from resellers.  Try not to buy a store demo lest you get one like my detector.  I went to the link in previous post and then did a search for some of the detectors listed and found them on ebay/amazon/???.
Many used one but many new or never powered.

(You need to Login or Register to view media files and links);

Solotripper beat me to it and I like his better.

I took another look on E-Bay and found the Humminbird model I have the PiranhaMax 165.

(You need to Login or Register to view media files and links)

It doesn't have the GPS but it's cheaper.
I only used it one time but it was easy to use and the screen is large and view-able in bright sunlight.

I don't know enough about this to pick one brand over another.
I guess you could read reviews and ask around.
I did that before I bought it and it was like the Chevy-Ford debate. It all depended on how they worked for the user.