Thinkpads are inexpensive, durable, and upgradable. Goes great with Linux. This article will show you what can be done with them.
Flash custom bios, this unlocks MSR so you can use software overclocking. Programs for Windows include: CPU-Tweaker, SetFSB, PhenomMsrTweaker, CPUMSR, RMClock, setPLL, WBX Tune, and various motherboard-specific tools. Linux has msr-tools which provides wrmsr and rdmsr, you need to know how the MSR works for your CPU.
eBay price: $40-70 https://www.ebay.com/sch/i.html?_nkw=x220&_sop=15&_blrs=recall_filtering
Specs: 12.5" 1366x768 LED, i3-2310M-i7-2640M, 16GB PC3-14900
Mods: 1920x1080 screen, X230 heatsink, BIOS mod