Is this fairly common? DH and I really do want to make aliya but it probably won't be for a couple of years. However, we have friends who bought apartments before making aliya (one of our friends bought three years before moving). Is there any benefit to doing this except having a home to go to when you get to Israel?
If you're worried about real estate prices going up, that would be a reason to buy now. But until you're there and know for sure where you want to live, I don't think it's a good plan to buy now, and from across the ocean. I also think that new olim get certain benefits when buying that you would be giving up by buying now. You could check with NBN for details.
I know quite a few people who have done this. The only advantage I can see is that you have a home as soon as you land. Otherwise it can be a huge headache to manage everything from abroad. And you might not like the neighborhood you bought in.
If you buy after making aliya you pay much less for tax
This depends on the cost of the house.
Yes, real estate prices keep going up here. If you are absolutely certain about where you want to live, then buy. I think for most people it makes sense to save up before aliya and rent when you first arrive.