I can't speak to standalone filters, but can recommend a heater. After much research, I decided to get this one based on reviews and reading this forum:
Hydor THEO 25W Submersible Heater
I've had it up and running for about 3 days now so I don't have any long-term impressions about it. However, my short-term impression of it is very good:
- Solid and feels durable
- Temperature is adjustable
- Seems accurate and responds well to adjustments
- A light comes on to show you when it's heating
- Can mount anywhere in the tank (i.e., horizontally if you prefer)
I've been fiddling with the settings on it over the last few days. Unlike some reviewers I haven't had trouble adjusting it while it's in the water. I'm most impressed with how accurate it is to finicky tweaks.
It's designed for smaller aquariums, so would fit your application nicely.
Hope that helps