Okay, first off, this is just end-of-pregnancy-panic setting in. You're about to enter the great unknown and you are, naturally worried about it. So take a deep breath annnnnnnnnnd r-e-l-a-x.
It's NOT going to be as bad as you think, it isn't, I promise. I've done it twice and have survived!
I would say your first plan should be to get a double buggy. This one is lightweight, narrow (for a side by side)comes with everything you need and won't break the bank. Maclaren do one very similar but about £100 more expensive.
Don't forget being pg is very different to being postnatal, especially after the first few weeks so it won't be as much of a physical struggle as it is at the moment. (this is where you tell me you're having an elective cs, isn't it!)
If you don't get on with the Baby Bjorn your options are to try a different type of carrier such as an Ergo or Coorie or just abandon that idea and know it's not an option.
The main things you have to remember when going out is to keep the changing bag stocked and ready so you can get out of the house quickly and to go when baby has just fed as it's more likely to fall asleep in the buggy.
Spend the next couple of weeks finding some short walks you can do just for a bit of fresh air and so ds can run off a bit of energy, even if it's just to the shop or park, or even a circular walk round the streets. Then when you get a bit more confident you can go further and for longer.
I found our local shopping centre a godsend when ds2 was born - he was a winter baby - if the weather was terrible we would wander round it and i could bribe ds1 and dd to be good by promising a play in the ELC!
Have you got any friends with similar age children who could take ds1 out for a while, to give you a break and entertain him?
Also, soft play places, especially during the school day are great because the older one can play and you can feed, etc lo as you need to.
It's taken me ages to type this so i expect I've x-posted with lots of other people but hope soem of it's useful and good luck - you'll be fine .