I have an Aqueon and Cobalt heater and like deelee said the aqueon is 3-4 degrees higher but it is pretty steady regardless.
The cobalt though is spot on all the time but i only have it in a 5 gallon for it. It's a little more money though but it's just really accurate and steady.
If you want cheaper I think Aqueon is the 2nd best and marineland 3rd Based on whats been recommended to me and ratings online..
But cobalt has the best reviews online.
Any of those three are good though overall.
Never buy a preset heater though those to me are just a waste of money because you can't control the temp and they have a knack for malfunctioning and overheating the water.
To figure out size and model i'd go to the cobalt site. And just get the same size or one above for the other brands. Buying a higher watt heater despite the extra cost initially means it takes less time to heat the water to the desired temp which could be a desirable feature but it's up to you. they all work pretty well at the minimum recommended size too.
Plastic ones are nice too because they have no chance of breaking on you but the glass ones are usually shatterproof and shockproof (atleast the cobalt one is) so it's not likely to break unless you deliberately smash it lol.
I'd assume you need a 150w.
Also agree with Deelee on putting it in front of the filter return it works much better that way. You could also put it horizontally in the middle of the back and it'll have the same effect. It depends what will be easier to hide it and if you're bothered by sticking your hand that far down in the water to adjust it.
https://cobaltaquatics.com/product-category/heaters/