The speed 6 engines will all need a rebuild at some point but thats now coming with a 3 year warranty - some I know of have done 100,000 plus since a rebuild. The AJPs not so bad reliability wise but not cheap to rebuild if they do go pop
The servicing and spares on a cerb are expensive and the chassis will need a refurb at some point but this is somewhat offset with lack of depreciation and with the fibreglass body there is no need to worry about rust as you would with most 15 year old cars. Mate of mine has just spent mearly £3k sorting out the sills in his 944 turbo and now has to get front wings done as well !!
If you want a cheaper way into TVR go down the Griffin/Chimaera route, engines pretty reliable cheap to service and look after and deprecation free as well The Griffins especially are rising in value every year