I recently tried skating in a $3.50 used pair of boots at a thrift store that were too big (9 instead of about 6), just to see if I could do it. I used all the right types of padding - insoles, tape, moleskin (on the sides), so I had full contact everywhere, and did it so I was well balanced.
But I kept stepping on the back of the blades, because they were longer too, and extended too far back of my feet. I've never been happy with freestyle blades cuz they are too long, but it was a much bigger problem because 3 sizes difference was simply too much... You can compensate pretty well for about 1 size too large if you work hard at proper padding. I considered cutting off the tails of the blades, but didn't bother.
Didn't help that those boots were so broken down and floppy they felt like soft moccasins. I used a lot of tensioned hockey tape tape (expensive) to stiffen them, and eventually had almost completely covered the boots with tape, but it didn't work as well as "real" boots.
You could make do if you are careful, and don't ask too much of yourself. It really is important to make sure your feet below the ankles can't move around inside the boots, and that you do have roughly equal pressure everywhere below the ankles. You also need a little pressure on and above the ankles, though you should use something like band-aids or tape so you don't get blisters there.
If, like my thrift store boots, they aren't very stiff, or you don't succeed in making things tight, you could get a major injury, maybe a broken bone, that could cost you much more than new boots. Likewise, if the boots are too stiff for you, they may cause you a lot of pain.
I'm not clear how experienced a skater you are. If you haven't skated much, you may not be too clear on how everything should fit, and getting boots from a good fitter that are for something near your skating level probably makes sense.
Boots and blades that fit your feet well will make everything easier. But it's up to you.