It would help to know what you are planning on using the bike for; commuting, group rides, centuries, triathlons, or just exercise.
Also, how much work are you willing to put into fixing it up. If you need to start buying tools, the cost quickly adds up. There is probably a co-op in Lincoln that will allow you to use their tools.
When you say you need a 21", I assume you are basing that on your mountain bike. You really can't compare the 2. While I ride a 21" mountain bike, I ride a 60 to 63 cm roadbike, which translates to around 24".