As above replies it really depends what tunnels you intend to go in.
Id say No 1 is a helmet, they are not expensive & even if its drains with little chance of falling debris you can easily knock your head.
Head torch makes things 100 times easier but you can do most stuff with hand held.
Personally I think waders are a waste of time & make moving difficult (though I dont do drains so may feel different in sewerage) I use wet suit socks & accept that Im getting wet.
For the same reason walking boots are often better than wellies cos u just end up emptying them every few mins, its quite common for mine explorers to drill holes in their wellies.
Oversuit if you are concerned about ruining clothes & gloves though I usually end up taking the latter off in 10 min.
u may need a kit bag & darren drum if you are doing really wet stuff (or just get a £150 Ulefone, best thing I own
)
Most importantly do your research, others have mentioned 4x gas monitor (5x is really the one to have as 4x dosent check the silent killer) but these can be thousands, if you know the type of tunnel / mine you are entering 95% of them dont require one