Choice is yours, if you want diesel and you feel that is right car for you.. go for it. Few things to consider: Obviously, XE20 (that is IS 2gen. as a whole) is really good value overall and comprehensive equipment by default. When considering used MB's and BMW's with same equipment they are at least 2x as much. Even XE30 has less cool stuff in "base trim" (e.g. keyless entry or what lexus calls "Smart entry" is an option for a lot of £££). I am still amazed with all gizmos I have in SE-L and that is third IS (XE20) I own. Few more things to consider:
ALE20 (200d/220d) has no auto option. For me that was main issue and manual for this model is not great e.g. maintenance costs are actually higher and it is less reliable e.g. clutches likes to wear and slip. While on the other hand v6+auto is one of the best sorted options (at least for relaxing driving).
Diesel engine is as well not the best ever (especially 220d), many reliability issues. 200d seems to have addressed majority of 220d issues, but still far from perfect.
Finally, driving style and location - XE20 is motorway car, especially ALE20. In city you will get DPF problems very quickly, as well as clutch (might depend on person's "manual" skill). In other hand there is nothing better than autopilot on motorway (especially if you have radar one... which correct me if I am wrong was never an option with manual?!), unless you have a diesel and need to change gears down when going uphill.
If you are really in need for a city car then why not CT (A10).. it is by far the best looking hatch for me.. maybe only new mb a-series with amg pack comes close in terms of looks.
In conclusion IS 250 (or otherwise GSE20) auto is much better option. It is still quite efficient - sitting in London traffics all day my long term AVG MPG is 25.2, but you can easily get this car to 30-35 on mixed cycle. Other thing as mentioned here - http://www.lexusownersclub.co.uk/forum/topic/72447-increase-your-mileage-in-few-steps/ - IS is heavy car and that is something you shoudl use to get good fuel economy, what is applicable for 220d is aplicable 250 as well. Basically, you will save little bit with ALE20 at the pump, but will spend just as much on maintenance, while at the same time it will never be as wonderful to drive as GSE20. As well it doesn't seems like SE-L's cost much more than SE's hence it is all about being little lucky and patient until there is car in the colour option you looking for. Sent from my Galaxy S6 edge using Tapatalk