Good questions.

The main thing is the anatomy of the area and the ammount of remanining
bone. It may or may not need to be grafted.

In the long term implants are probably your best solution but as you
said there is some expense. Get a consult with a surgeon, if you have
x-rays that you can post that would give us more to go on.

As an temporary measure if you cannot get implants right away
you may consider an interim partial denture (commonly called a 'flipper')
this would keep teeth from shifting while you saving up for the implants.

As far as #17 goes, would need to see a picture to even venture a guess.

Best wishes,

Teeth 18 and 19 both had root canals and crowns about 10 years ago.
Last year, 19 cracked below the gum line and had to be extracted (that
was fun).  Last month, 18 abcessed (even more fun) and was pulled as
well.  What to do now?
I've been told I have bone loss in that same area.  If bone is grafted,
do I have a pretty good chance of gaining enough strength to hold
implants?  If I leave the space empty, is 17 next?  My preference would
be to make the best choice, in terms of the long run, even if it means
greater expense.