1. You need to be clear as to what you want:
1.1 A sealed central heating system, with no Feed and Expansion (F&E) tank in loft and an expansion vessel geally inside or near the boiler with pressure relief valve, pressure relief pipe terminating outside and means of re-pressurising. AND / OR
1.2 An unvented hot water system. No cold water storage cistern (CWSC) in loft, suitable unvented hot water cylinder with either internal pressure vessel or external pressure vessel, required safety devices including temperature and pressure relief valve (TPRV) with tun dish and pressure relief pipe in accordance with regulations.
2. Most reasonably modern boilers can be used in a sealed system, the boiler installation manual should state whether or not this is possible. Only risk is that the pressure in the system may cause / exacerbate leaks in fittings / pipework requiring them to be fixed. I'd suggest within the scope of an experienced DIYer with plumbing experience.
3. Unvented hot water systems may only be fitted someone with a current "G3" registration. There are strict rules for their installation, and the installation must be notified to your local buildings control office. Normally done by the registered installer.
4. Price cannot be forecast without a site visit. Such a visit is necessary in any case to determine whether the available water pressure and flow rate is sufficient for an unvented hot water system. However, as a starting point I'd work with (all plus VAT):
4.1 Convert existing space heating system to sealed system, probably around £750 plus time and materials required to correct any leaks discovered.
4.2 Install a new unvented hot water system, probably start at around £2,500 (including cylinder and controls) but could easily double depending on site conditions and pipework requirements. Sensible to budget around £100 per appliance for any needing changes / replacement because of higher pressure. (e.g. WC cistern inlet valves).
