I’m wanting to transition my daughter to a big bed, our only spare bed is a queen size and it’s slightly raised off the floor and I’m not wanting to put the matress directly on the floor as I worry about mould. Is it ok for the bed to be pushed against the wall at this age? I also am unsure when to introduce a doona and pillows and a top sheet etc? What else can we do to make it as safe as possible for her

How old is your daughter?
The general Safe Sleep recommendations to reduce the risk of SUDI (Sudden Unexpected Death in Infancy - SIDS & Fatal Sleep Accidents) are predominantly related to babies less than 1 yo.
However, when transferring baby from a cot to a bed, there are of course, other risks.
Its important to weigh up the risks & the potential for injury.
Have a look at the Red Nose “Cot to Bed” brochure. https://rednose.org.au/downloads/RN0407_22_Cot-to-Bed.pdf

Is it possible to get a single bed or toddler bed that is low to the floor, with specially fitted rails?

As far as using a doona, &  pillow is concerned, the general recommendation is:
Red Nose recommends waiting until the child is over two years old and no longer sleeping in a cot or portable cot before introducing pillows into their sleep environment
https://rednose.org.au/article/pillow-use
https://rednose.org.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows
A top sheet can be used. Sleeping bags may now creates a risk of falls if used when toddler is in a bed.
Also, check what areas of room & House your child will have access to now they are able to wander.
See also Cot to bed brochure linked above.