You described what you want very well, so kudos on that! It helps me give you the best advice possible as well.
The thing is, convertible - single to double prams are, usually, on the heavier and also more expensive side if they are quality ones. I mean, to be able to withstand the weight and wear of two children and different terrain use, it HAS TO be sturdy and well made, and that costs money. A good example would be a Quinny Hubb which I had for a time and was in awe of how large it is - but it did accommodate even two large toddlers, which is a rare thing with a reversible stroller (and a double at that).
My best advice here would actually be, yeah, the Uppababy Vista because of a very well-made chassis - but yeah, a quality stroller means more money spent and you wanted a less expensive one. But I would still suggest keeping this in mind and maybe look up a used one for less - it will be ecological, economical, and a practical solution to your situation.
Next in line, at least in my eyes, would be a Baby Jogger City Select Lux because of its quality as well as various cool options like a bench seat for the older kid. I tried the bench seat and loved it because it is suitable even for older children (like a 6yo) when they can't walk anymore. Also, this one is rather compact as the seats fold, and it can fold with both attached to an OK-sized package. This one you'll get for quite a nice sum nowadays; just be aware that in the double mode (as almost all single to double prams are), the maneuvering is simply worse.
If OK with pre-loved options, I will also suggest a Thule Sleek as a quality option and one that not everybody has. Again, not the cheapest model and not the most compact either, but a spacious one and a unique one, with many practical functions.
Ans I can't forget to mention the Bugaboo Donkey 3 that is just the best in some features at least, and a very high-quality one, light enough even, just not the most compact. If you found it somewhere in classifieds, I absolutely recommend at least go and check it out. Many mums are incredibly happy with this one.
The Mockingbird stroller, I feel, has quite a lot of marketing and not always honest reviews involved. I mean, as a kind of a copy of the Uppababy, and such a low price tag, it is never going to be as good, high-quality, or as spacious. BUT it IS rather cheap, and so, it might be worth your attention if the price is the higher stroller priority of yours. If it is so, I say go for it, why not. But do count on the price being reflected in quite a few areas of the stroller, and of course, a soft suspension is not what it's going to offer. But, again, the performance seems OK and even GOOD for the money. It is, in any case, not my favorite - I feel like designing is better than copying, and I am also quite disappointed about many low-cost prams and pushchairs out there. They simply last badly, and here, the manufacturing solutions, joints, and a one-hand fold with a tandem (!) make me pretty sure this is a cheap (not just price-wise) stroller model, so I personally would NOT recommend it.
One more thing. Most of the strollers mentioned are bucket-style seat strollers, which means they NEED a special wedged nest to be able to accommodate a newborn. A carrycot, however, would be a much comfier and healthier option, that is good to know.
Hope I helped and inspired you! Please do let this lay in your head for a bit, and go for it with your heart. It will be mostly for you and the baby.
And don't forget to leave a review once the stroller (and baby) comes to help other parents with their struggle! Good luck!