To display a single line from a large text file:

sed -n "${LINE_NUMBER}{p;q}" $FILE

The p command prints matching lines, and the q command causes sed to quit after the first match.

You can’t use q if you want to print a block of lines, so you’ll just need to ^C after the output is printed so that sed doesn’t keep needlessly reading the file.

sed -n "${START_LINE},${END_LINE}p" $FILE

If you want to replace a single line, you can use a similar trick (once you’ve identified the line number):

sed -i "${LINE_NUMBER}s/.*/${REPLACEMENT_TEXT}/" $FILE

It may be advisable to use single quote here, rather than double quotes + variable substitution, to prevent shell weirdness. Unfortunately, there’s really no way to tell when this command has finished (except to let it work through the whole file).