As a rough guide, if you buy quality generic kit from the likes of Woosh (https://wooshbikes.co.uk/) expect around £500-£600. A lot depends on the battery capacity. You can get similar kit from Yose (https://www.yosepower.co.uk/collections/e-bike-conversion-kit) probably a bit cheaper. Beware a lot of their kits are nominal 350w motors, which makes them not legal as a pedalec. If you decide exactly what bits you want you can go a lot cheaper from Ali Express or similar. Alternatively, you might go for a more upmarket system like Cytronex (https://www.cytronex.com/). This will set you back around £1000, even with quite a low capacity battery. Also, any spares are likely to be expensive, with the risk of their becoming unavailable. One of the lightest systems, partly because of relatively low power and low capacity battery. You will need to decide if you want crank, rear hub or front hub assistance. Lots about that on the forum. If you go for rear hub you will lose your nice rear wheel. If you go for crank you may well lose the front clanger. Front hub is generally less liked, but can be good on a tandem. More weight on the front wheel reduces the risk of front wheel scrubbing on gravelly hills, and you get two wheel drive with no need to compromise on gears and rear wheel. Swytch is a common option, but not generally liked here. Their expertise is marketing rather than technical or support.