Workshop build

I bought my shed from TUIN, a Dutch firm producing solid sheds from real timber.  Definitely not the matchboard stuff at B&Q etc.

A bit more DIY, but Colin Furze has a good shed build tutorial video:

https://youtu.be/LP67MAoihZk

It would need some windows though if using it for more than storage.

I fitted Pragmasis Torc ground anchors in my concrete floored block and stone shed (following a break in where the thieves had broken the body of a 5 lever mortise lock). Their website is worth a look, they also sell mighty chains and locks to go with them.

I also now have two paired mortise locks, which makes it much harder to lever the door as both would need doing together.

Ground anchors - the motorcycle parking at my local train station has this style of ground anchor. When it's been concreted in, it sits flush with the ground and looks like a very neat solution.

As far as heating goes, these babies are awesome.

https://www.ebay.co.uk/itm/Diesel-Air-Heater-12V-5KW-Air-Diesel-Petrol-P...

Firstly, get as big a shed as you can. 3m x 4m will be very tight - my bike shed is a little smaller than yours, and contains seven bikes, one tandem and various bike bits, bike tools and bike paraphenalia. There is no way I could do any bike maintainance in there.

On construction, mine is a log cabin type shed and this is very sturdy. It just sits on its foundations and certainly isn't going anywhere. The shed itself was faily easy to build, but the most difficult (and critical) part was to get the foundations right. These need to be flat and solid. Because mine had to go on a sloping site, I had to build a sort of raised deck to put it on. This works, but wasn't an easy job.

It is possible to supply power to the shed, but unless you have a convenient wall to run the cables along they will have to go overhead or underground. Neither is particularly easy, and there are regulations on both. I would get professional advice here.

If you haven't seen them, have a look at Robin Clevett and the SkillBuilder channel on YT, there was a recent series on a garden room which might provide some ideas for you. 3x4m isn't a lot of room considering what you want to put in there and do with it. Just storing 4-5 bikes can take up a decent amount of space - I used Steadyracks because they hang bikes vertically and swivel to minimise room intrusion, but you still need clear space to be able to put bikes in them. Trikes especially take a lot of floor space and it's not always practical to hang them as mine shows.

If you plan to hang the bikes vertically then  consider eave height. A lot of kit sheds are designed to have a gable height under 2.5m but the eave height may only be 1.9m, which does not give much leeway and if you want to stagger the hanging height to store bikes closer together without handlebar interference you might not be able to. A skillion roof so you can maximise wall height to hang bikes on one wall without exceeding the overall 2.5m height might be best.

I would stick to a timber frame building simply because even using a kit it's quite simple to add extra supports where you want them. Insulation is easier too as metal sheds have all kinds of thermal bridging to deal with.

Think about some serious thermal insulation, including the floor and ceiling, and decent ventilation. Very easy to build a summer sweat-box where you freeze in winter!

If you need to trim the hedge will you need to be stood on the shed roof? And will you need something to prevent the hedge frondles from rubbing through the edge of the roofing felt.