If you aren’t familiar with unicycles, maybe you have more infos to give about what you want to do (riding on the grass can range from flatland to muni, which is extremely different).
Do you already yknow how to ride a uni, and if not, what is your inspiration ? (It usually doesn’t come out of the blue).
Sounds like a bigger wheel, say 26“ plus, makes most sense, given that you‘d like to be able to ride it on grass. If you’re not really heavy you could get away with a cotterless (square taper) cranks, I guess. With ISIS cranks you‘re on the save side, but that leaves you with a used uni, given your budget.
If you can‘t find anything second hand I‘d have a look at unicycle.com Australia. A Trainer (steel frame, cottorless, no brake) seems to be the main option within your budget. I haven’t first hand experience, but from what I’ve read this should be up to the task. There‘s also the Club Muni, but at 24“ it‘s a bit on the small size in my opinion - though the wider knobbly tyre is a good choice to ride on grass.
Thank you for your advice! If I go with a wider tire to ride on grass, would it still be okay to ride on concrete? I’ll mostly ride on grass, but I might use it on concrete about once a week, so I’d like to know.
Of course it‘s possible to ride a road tyre on grass, particularly on a bigger wheel like a 29er. But if grass is the primary surface you plan to ride on, a wider tyre with knobs makes more sense in my view. No, it‘s not ideal on tarmac, but if you pump it up it rolls reasonably enough and (important!) can be steered well enough.
It is not difficult to ride with a knobby tire on concrete. These kinda tires are less rwitchy and I abuse all my municycles to ride on paved roads nowadays. Concrete is still easier than bumpy grass. A24 muni would be ok for around the house, but once you start riding more, you may want to go for longer rides as well. Then a 26er is more pleasant. A 29er might be too big to ride around the house with.
It’s good to know that it can be ridden on both grass and concrete — thank you very much for the information. Could you please recommend any specific models?
Tyres? Not really.
My muni is a fatty, so my experience is mostly with 26x4ish tyres. I had a 27.5x3 Duro Crux for one or two rides. It wasn‘t bad (others will disagree) but wasn‘t really more manoeuvrable than my 26x3.8 Surly Nate. Yet, it lacked the rolloverability of the fat tyre, so I switched back to my original wheel and tyre.
I have a Continental Race King in 29x2.2 on a different uni. It‘s perhaps a bit narrow for your intended use, but I think it‘s also available in slightly wider dimensions. I like this tyre a lot. Little rolling resistance, great agility, good grip. Only minor gripe: it‘s on the noisy side on tarmac. But that doesn‘t deter me from using it on my most used uni; mostly on tarmac, by the way.
I am a multi surface rider, pavement, crushed lime stone and gravel. I have over time rebuilt my 26 29 and 32 onto oracle frames from UDC. I would recommend and oracle 26 er it will be set up for muni stock but you can swap out tires quite easily. It is lightweight durable and versatile. 26 is a good all-around size, right now my 26er has a 2.5 inch maxis holly roller on it, a better multi surface tire than the duro wildlife. . I would check with UDC Australia.