dogs can be hugely rewarding (and make you want to pull you hair out at the same time). I have 4 currently. I agree that you need some means to pay for vet bills when they arise (and they almost always do outside of the planned-for annual vet expenses), but I would not go so far as to say you need a few grand in savings before you start.
i agree, fostering ccan be a great way to get your feet wet, especially if you want a border collie. they are super-high maintenance dogs that need constant jobs and interaction to keep them from getting bored and destructive. if you have the means to do this, by all means get the dog you are dreaming of. they are super-intelligent and will get themselves into trouble when they are bored... I have a silky terror who is bored very easily and drives us nuts because he is not always getting the activity he needs. foster agencies will also provide help with training techniques and problem-solving for the dogs they place with you. many will also allow you to adopt the dog if the fit is right.
I'm sorry if you have experience with dogs already and we are making it seem like you have never had any. I have had dogs since I was 5, but only recently had more rewarding experiences (especially running a pack). I was incredibly lucky with most of the dogs I have had in my life, and only had one with major behavioral issues that I could do nothing about (brain trauma from previous abuse caused a lot of unexpected aggression 4 years after I took her home). Also, keep in mind that working with someone else's dog (sitting for a friend) is very different than having your own. They build relationships and get into routines much like we do. Make sure to match the dog you get (or take in to foster) with your lifestyle. Some (individual) dogs do better with certain lifestyles than others. I try to pick dogs that have a moderate energy level and a calm demeanor because I don't want to deal with constant activity ad engagement with my dogs. I like to go for a few long hikes a week or every few weeks, but nothing too strenuous daily. A typical border collie would not be my ideal dog. even my silky is not my ideal dog as he needs more constant activity than I provide... we all suffer for it, but we are both a work in progress, so when I'm feeling better, he is more balanced because we do more both individually and as a pack... we are also working on his cat-hunting skills (namely squashing it before he escalates out of control)...
ok. I'm rambling. good luck!!