Get the best and warmest down bag you can afford. Something like a Rab or an Ajunglak. I have had my 4 seasons 900gm fill bag now for almost 20 years (reminds me, it probably needs a wash 🙂 ). Comfy to -12 and good for -23. I have slept in snow holes and in the Sahara desert. When it is hot just open up and use as a duvet, when cold, zip it up tight and wear your thermals inside. A good bag will last and last and should be considered an investment.
I would always rather have a bag that is too warm than suffer a night of being too cold (and I have suffered hundreds and hundreds of them in the past when camping in the past with shite bags.)
The advice on a good mat though is probably even more important than the quality of bag to be fair. Even the best bag is only as good as the insulation from the floor. And don’t skimp and buy a cheap air bed as they are shite too. A 10mm closed cell foam mat is infinitely better, but a self inflating mat with a foam core is better still. Quality counts here too and it really is worth spending on a good brand like Thermarest.