You can put heaters at a slant; lots of people recommend it. The reason is that the hot water rises up, fools the thermostat into thinking the whole tank is that warm and the heater shuts off. I can't quite see that myself as the water flow from the filter should sweep the newly heated water away before it has chance to rise.
But there are a few makes of short heater around. 25 litres needs a 25 watt heater. I have two - one Sera, one Visitherm. The Visitherm is just under 8 inches from the bottom of the tube to the top of the knob on the top of it. The Sera is a tiny bit shorter as that had a wheel to alter the setting rather than a knob. Though it sounds as though they'd still need to be on a slant in your tank.
As for filters, I'm not sure. The one I have in my 25 litre betta tank, and the 25 litre quarantine tank isn't made any more. For a betta, you need a filter with a gentle flow (and the flow can be turned down on mine).
Be careful with small filters, some of them have awful media inside. My small filters are Hagen Elite Minis; the replacement for them is the Marina i25, a good example of what I mean by awful media. The Elite mini has just one sponge and is fine for a betta. The i25 has a cartridge containing zeolite and carbon, that's all. Zeolite should not be used in any filter, in my opnion, and I don't use carbon.
Maidenhead Aquatics (they are on-line as well if there isn't a branch near you, though you'd be better seeing it in the flesh to make sure it would fit in your tank. ) sell the
Aquamanta IFX 50 which is what I'll probably get if my filter stops working. It says it has an adjustable flow rate and it shows a spray bar attached. It is easier to "adjust" a filter with a spray bar by covering the spray bar with a piece of netting, or even a section of a pair of tights. But it does say that it performs mechanical, chemical and biological filteration - the chemical could mean carbon or even zeolite. I would use neither, just add a bit more sponge.
I'm not very good with plants so other people will need to advise you there. But I can tell you that whatever decor you put in a tank with a betta needs to be smooth. Rough decor or sharp edges can damage a betta's fins. There is something called the tights test - run an old pair of tights over the decor and if they snag, it will potentially damage a betta. Small rough areas can be sanded smooth, but that is impratical with large areas. And bettas are curious fish; they will squeeze through holes to see what's on the other side. They will try to get through holes that are too small and get stuck. If your chosen decor has a hole that a betta could get stuck in, find some way to enlarge it, or block it.
And on a slightly different topic, in fishkeeping, it's American gallons (except for some British medications, which is why it is safer to use litres for working out the dose). It is always US galls when sites talk about tank size for stocking. So 50 litres is 13.2 US galls; 25 litres is 6.6 US galls; and 20 litres is 5.3 US galls.