Hello and welcome aboard.
It all depends upon your time and abilities in addition to some funds. There are a couple of options that I see and probably many, many more.
One is to forego the internal modifications and get a stand alone shower tent. You can use solar showers or the portable water heaters like the Coleman Hot Water On Demand water heater. You'll need a method of collecting the water runoff. Some folks use a household water heater overflow pan for that. We had a shower tent and used it on one trip then put it on the shelf. Works well for some folks.
For the shower/toilet combination units, it is tough to figure out if uses just their standard cassette toilet plus a specific base, i.e. shower pan or if it is a specific model cassette toilet that is fitted for the shower pan. You might just need to find the base that matches your model of toilet, which is probably the C400 series. A complete setup, pulled from an old PUP, sold for $300 with local pickup on ebay recently.
It wouldn't be too hard to build a shower pan yourself. There are lots of images that Google provided for inspiration. I did a Google search and found
this for starters.
And using a laundry sink tub basin is another popular DIY option as they can fit inside the base cabinets for seating and then you're attaching a curtain to the ceiling, adding a faucet with detachable shower wand.
Everyone's experience and wants are different. I really wanted a shower and toilet in the first PUP we purchased. For my wife, it wasn't high on her priority list. That was ultimately the reason I sold it and bought the current PUP. Being a lot bigger inside and tons of amenities sure didn't hurt either.
Well, since getting one with a hard wall spacious toilet and shower (almost double the area provided by standard PUP shower/toilet configurations), I'm the only one to have used it and that was once. The toilet is invaluable, the shower not so much. The hard walls of the bathroom are dang useful for the obvious purpose but came with a mirror on the outside, makes an awesome place to put towel hangers, organizers, etc.
I would make the following suggestion: either buy the unit that you want with everything in it, which is called "buy your second PUP first" instead of settling for a great deal, OR buy one and immediately buy the parts to make the necessary changes. If not, probably you'll limp along for a few years and finally give up and sell it or just not be satisfied if you don't happen to ever get around to the major modifications.
BTW, I love doing mods to my PUP. It's a hobby in itself. I have not done any structural reconfigurations yet but lots of creature comfort or usability upgrades. See
here for some of what I've done. Others on here done quite a few.
