An offboard charger made with server power supplies would be pretty interesting.
The cheaper supplies are not very power-dense and probably not very resilient to bad weather. Fortunately neither is a problem when charging at home, provided you have a weather-tight space to charge in.
One thing to consider is that the incremental cost to increase charger power goes up as you increase power. Big gains initially, but later incremental gains are relatively small.
Example: Let's say you add strings of two
TDK FPS-1000-48 supplies in series.
$170 per 2 kW on ebay.
Let's say you went on a 40 mile ride that needed about 5 kWh to recharge .. up to 95%.
The 1.3 kW offboard charger would require about 3.8 hours.
1 string of TDK supplies would increase charge rate to 3.3 kW, shave 2.3 hours from the charge time (down to 1.5 hours).
2 strings of TDK supplies would increase charge rate to 5.3 kW, shave an extra 0.57 hours from the charge time (down to 0.94 hours).
3 strings of TDK supplies would increase charge rate to 7.3 kW, shave an extra 0.26 hours from the charge time (down to 0.68 hours).