1992 Subaru Legacy L AWD non-turbo non-ABS wagon

I've been looking around to see what the parts would cost for fixing
my suspension ails.  The bumps are hard on my butt, much harder than a
few years ago.  It's like the springs are sagging so I'm bottoming out
or maybe the struts are failing.  It rides okay and handles in the
turns okay but the bumps are jarring my fillings loose.

I found http://www.mileaautogroup.com/ for one online store (there are
loads of them but seem to be front ends to the same few catalogs) but
I don't know yet if they have the best prices or are a reliable store
(or what brand parts they are selling).  To do the front and rear
suspensions, I got a shopping list of (numbers rounded):

Front:
   Coil spring (right); qty. 1, $63
   Coil spring (left); qty. 1, $63
   Strut (right); qty. 1, $105
   Strut (left); qty. 1, $105
   Upper strut mount; qty 2., $53 each

Rear:
   Coil spring; qty. 2, $63 each
   Strut (right); qty. 1, $108
   Strut (left); qty. 1, $108
   Strut bumper; qty. 2, $24 each
   Strut dust shield; qty. 2, $14 each
   Strut mount (right); qty. 1, $86
   Strut mount (left); qty. 1, $86

If I got all the parts, the front would cost $428 and the rear would
cost $549 (for a total of $977 for all four corners).  Now I see why
the shops were quoting their prices around $1200 (I forgot to ask them
if it included the alignment).  What I'm wondering is, if I do this
myself, how to determine if I really need the coil springs and strut
mounts.

I've been recommended to go to a body shop to have them measure the
ride height (unloaded) which would indicate if the springs were
sagging?  Is that true?  Is it an adequate test?  All 4 struts were
replaced 5 years ago but not the springs so the springs are the
originals that are 14 years old.  Bouncing on the corners won't tell
me how good are the springs, just maybe how good are the struts.

When taking it apart, how would I know if I need the strut mounts
(i.e., the plates at the top with the sleeve bearing)?  Could I tell
if they were worn?  Could I tell if they were loose (i.e., so the top
of the strut wobbles around)?

Could be to do a decent job that all these parts need to be replaced.
At that point, I'd rather pay the extra $230 (both front & rear) to
let the shop do all the grunt work.  But if I don't need the springs,
strut plates, and strut bumpers, I'd chop $232 off the fronts and $346
off the rears (or $578 total saved, or a total expense in parts only
of $477).  The difference between $1200 (parts & labor) and $477
(parts only), or $723, is enough to make me plan on doing them myself.
I'm not sure what the shop was going to replace (i.e., if they might
try reusing old parts) or if all the parts would get replaced.