Here's some interesting articles about replacing the solenoid contacts/etc on the Nippondenso starter that's used in DC minivans. I've never done any biz with Nations Auto Electric, so don't take this as an endorsement. Their info's pretty good on rebuilding the solenoid. I buy from a starter rebuild shop locally.
Starter Maintenance
Nippondenso Starter Parts & Repair Kits
To determine what length of extension to buy, measure the distance with a steel tape and then get an extension/s that will fit.
I got a set of
wobbly extensions from Harbor Freight that I really like. Wobbles are better than standard because they allow a 16* tilt of the extension within the socket. Sometimes it's just enough movement. They are more ridged than a swivel and won't lock up.
Sears Craftsman also has regular extensions, in sets and by the each, as do all the box stores. For the home mechanic it's hard to beat Craftsman tools. Shop for a tool box set on sale.
Extensions are a lot like shop clamps...do you ever really have enough?
A good tool, you buy once. A cheap tool, you buy again and again and...
As always, good luck with your project!