you should be looking around the £6k mark for a quality system – My preference is LG and Hyundai who are both going to be here for the long term and have excellent ‘low light’ panels. A hight quality European Inverter is always my preferred option.
The Guarantee issue is I believe quite important. All MCS accredited Installers must under RECC rules offer an Insurance Backed Guarantee – However they give installers the option to offer 5 years. You will find that most reputable installers will offer 10 years.
In real terms this business is always going to be governed by Political will and to be honest the FIT may not be here in say 7 years and this will lead to many installers leaving the business so it is vitally important that alongside getting all your product guarantees registered you also get an independent 3rd party insurance backed guarantee.