It all depends on what you are looking for.
Here's our story, take from it what seems helpful:
My son was climbing out of his crib by about 12 months... seriously. (He was walking at 10 1/2 mo). So he was on his crib mattress on the floor for a bit when he first demonstrated his new "skill". Then we received a hand-me-down twin bed from my mom's attic. We pushed it into a corner and used a bed rail on the open side. He was okay with that. Not long after, I was pregnant with #2, and we needed to make longer term considerations... like visiting grandparents on both sides of the family at the same time, etc.. so we moved him into a "new" room and bought a full size bed. So extra guests would have a place to stay when the "guest" room was full... and for future sleepovers of kindergarten buddies. :)
We put guard rails on both sides of the full sized bed, and he was VERY excited about that.. he was about 32 months old at the time. No issues with him getting up out of bed or anything like that - all of that went on when his mattress was on the floor.
Honestly, even though the twin was really cute, and it probably helped with the transition (since a full sized bed wouldn't have fit into the nursery he would have had to change beds AND rooms all at once).... I was physically much more comfortable with the full sized bed. It made it much easier (particularly since I was pregnant) to lie on the bed to read bedtime stories and things. I could even lie down with him at his naptime if I felt like it. A single bed doesn't have the surface area to spread out and be comfy for stories, games, etc.
So a lot depends on the arrangement of your home and future needs. My son (now 11 yrs old) still has his full sized bed, and it is quite handy (even with another designated "guest" room) when my brother's family of 6 comes to stay with us. We use his bed as the "gathering spot" for bedtime prayers still, too.
I did the mattress on the floor out of necessity for a short time, and though it is ok, I wouldn't plan that for a lengthy solution. It isn't as wide as a twin, and they WILL roll off it; while they won't get HURT, they WILL tend to WAKE UP during the night. Not somewhere I'd want to go when they are good sleepers otherwise.
I ended up getting an inexpensive used toddler bed for our daughter, and again, while it was cute and efficient, she moved up to a twin sized bed before her 3rd birthday. So, I would guess that your 22 month old would be in one for a year or less... consider that time frame when you are "shopping" your options. Also, unless the toddler bed comes with the rail attachment as part of the frame, most of the regular bed rails (the $20 variety) will not work properly with a toddler bed.
You are right to want to "nip" the co-sleeping before it becomes too ingrained a habit (unless you plan to start doing this all the time long term). So even if you need to wait a few weeks before you can get a new bigger bed sorted out, go ahead and put the mattress on the floor temporarily so he doesn't get too attached to nighttime company. :)
Good luck.
P.S.
Whatever your bedtime ritual is, don't change a thing! Everything should happen exactly the same (except maybe in your son's room now). It is great to let him help with a few choices (maybe ask him to choose between 2 sets of sheets, etc), and get him excited... but not SO excited that he thinks that all the "old rules" are out the window. ALL NEW stuff can be scary to kids! That will be one of the keys to keeping him on track. Bedtime is still the same, he will just be sleeping on a different bed (in a different room?). That is the reassurance he will need from you to make a smooth transition. We let our son get excited, but at the same time, tried to walk the fine line of not making it TOO big of a deal. That can also bring on stress, worry, and wakefulness, (think about how kids can't go to sleep the night before school starts, or on Christmas Eve)...