Which prescription diet best for chronic constipation/lazy bowels?

My 1.5 year-old male tabby recently had a bad bout of constipation. Toby
did not poo for over two days and a vet check-up found a substantially
distended colon. The vet immediately put him on Propulsid (bowel stimulant)
and Duphalac (lactulose syrup) but he still refused to poo for another two
days. The vet finally administered an enema under anaesthesia, which
thankfully helped empty Toby's bowels almost immediately.

While x-rays taken before the enema showed a distended colon, it wasn't
conclusive enough for a megacolon diagnosis. The vet has now put Toby on a
mixture of Hill's Prescription Diet w/d dried and r/d canned, and reckons
that Toby will also have to be on Propulsid and Duphalac for the rest of his
life. [Toby was also recently diagnosed with struvite crystals in his
urine, which necessitated the change of diet to Hill's w/d and r/d.] While
I'm okay with Toby having to be fed a fairly expensive prescription diet for
the rest of his life, I'm not sure I am prepared to continue paying my vet
the exorbitant prices charged for the two drugs. I did some research and
discovered that Duphalac can be had for as little as AUD$12 per 500ml bottle
at pharmacies in Australia without needing a prescription - in contrast, my
vet charges me AUD$25 for a mere 25ml bottle (which lasts less than a week).
I am, of course, a little peeved at the obvious profiteering going on here.
But that's OK, since it is something I can get around by getting the
Duphalac myself. Propulsid is a more complicated matter - it is probably a
prescription drug, which means I can't buy it over-the-counter at
pharmacies, and my vet charges me AUD$65 for a wee little 25ml bottle. I
also found out that propulsid has been withdrawn from the pharmaceutical
market for humans because of allegations that it caused the deaths of
several young children. So, quite apart from cost, there is the question of
whether long term use of Propulsid could harm my little boy.

So I am at a dilemma - should I risk stopping the use of the drugs and rely
on the higher fibre combination w/d and r/d diets to keep Toby's bowels
moving? What if I leave out the Propulsid and just rely on the Duphalac to
keep Toby's stool soft/moist? Is a higher fibre diet (the Hill's w/d and
r/d) necessarily better for chronic constipation or is a low-residue diet
the way to go? Is canned pumpkin safe for a cat with a potential struvite
crystals problem?

Thanks a million to anyone who can shed some light on the above. :-)

Devlin
Perth, Australia